1005 days. 73 episodes. And counting. That is how long it has been since Diamond Tiara last had a non-background appearance; and that was the same episode in which she was redeemed.

For comparison:
Luna- Redeemed in episode 2 and next appeared in Luna Eclipsed. 30 episodes and precisely one year apart.

Discord- Redeemed in Keep Calm and Flutter On and next appears in non-flashback in Princess Twilight Sparkle. 308 days and 17 episodes later.

Starlight- Redeemed in The Cutie Re-Mark and next appeared in The Crystalling. A mere one episode and 119 days later.

Trixie- You could argue she was redeemed (sort of) in Magic Duel, but for these purposes I'm gonna say No Second Prances. After that episode she appeared next in
To Where and Back Again, 175 days and 19 episodes later.

The only villain not to appear after redemption for a longer time than DT is Gilda, for whom there has been a 1153 day, 83 episode and counting abcense. However, she did have a direct impact on The Fault in Our Cutie Marks by sending a letter to RD via Gabby 476 days and 37 episodes after redemption, which is more than DT has done.

So the question is, why has DT, unlike her redeemed peers, not been allowed to do the slightest meaningful thing in over 1000 days?

The way I see it unfortunately it seems they have written her out of the show. I dont think the current writers know what to do with her character (same for silver spoon). That seems to be a trend though and probably why this will be my last season watching mlp.

6512726
Luna - major villain, important character, adult and powerful mage.
Discord - major villain, important character, adult and powerful mage.
Starlight - major villain, important character, adult, a powerful mage and de-facto part of M6 (M7?).
Trixie - yes, she not as important as others here but she a friend of Starlight.
DT - ...I like her but let's face the truth - she a minor child protagonist of CMC storyline and side character with little importance.

6512726
When has Silver Spoon gotten a solo-ish role like Diamond Tiara got in Ponyvilel Confidential? Not yet.
When has Lightning Dust ever been mentioned again? Not yet.
How about perhaps inviting Diamond Dogs to the School of Friendship? Nah.

There are tons of things the show hasn't picked back up in a long time. You might notice that all of these things, including Diamond Tiara, are minor parts of the cast at best.

In spite of the length of time and amount of episodes the show has, including all your counted days, the show is primarily about the main cast and certain recurring support characters. They aren't on some kind of rotation as you inferred. There isn't some kind of time limit between uses.

Each M7 (M6 before Starlight) gets a roughly equal share of episodes with her as the main character. They all often show up in supporting roles elsewise.

Each season has 1-3 Spike episodes, 1-3 CMC episodes, 1-3 Discord episodes (except S1 for obvious reasons), and 1-3 Princess Episodes. Now that Spike and Discord tend to be in the same episodes together as friends, their episodes are often combined. Counting as one for each.

S3 was obviously weird on the counts but still had its allotment from that second group (If you count Sleepless in Ponyville as counting for a Luna episode and/or MMC as a Celestia episode).

The problem with Diamond Tiara is that she was only ever used as a supporting character (antagonist) for the CMC, and was therefore only ever relevant in some CMC episodes. Redeeming her to be a friend didn't change that it just means that if she's used in CMC episodes in the future, she may well be an ally/friend supporting character rather than the antagonist. Why haven't they done that yet? No clue. I think she would have been a neat part in Marks and Recreation or Forever Filly.

Also, to clarify, Crusaders of the Lost Mark wasn't a "Diamond Tiara episode." Her role was pretty prominent, but she served as a catalyst for the CMC getting their cutie marks. Helping her with her cutie mark issue was what got them their marks. It wasn't the start of a story arc for a redeemed villain, it was an event along the way in the CMC's arc. The CMC have continued to have episodes following that change, and adapting to no longer crusading but instead helping/counseling others.

The writers haven't exactly written her out of the show, they just haven't had reason to include her in the stories they're writing. None of "her" episodes (even Ponyville Confidential) were really "her" episodes. Just like Griffon the Brushoff wasn't a Gilda episode, it was a Rainbow Dash/Pinkie episode. Boast Busters wasn't a Trixie episode, it was a Twilight Sparkle episode. And so on.

Luna and Discord we get to see because they're prominent enough characters to get an episode or two per season. Starlight we get to see again because she's one of the main characters. Trixie we get to see because she's often in Starlight-focus episodes (along with Sunburst), as one of her regular supporting characters. The same way we get to see Maud in Pinkie/Starlight Episodes, or various fashion ponies in Rarity episodes, Wonderbolts in Rainbow Dash episodes, and so on.

Once you correctly identify what kind of character each of these ponies is, it makes sense. We may like Diamond Tiara a lot, but be honest about what sort of character she is in storytelling terms in the show. Leave it to fanfic to promote her to main character. You won't be as disappointed.

I take it the mlp mysteries comic books are non-canonical?

The latest mlp Mysteries featuring the CMC as junior detectives to solve the mystery of a fire that breaks out at the old ponies' retirement community. One of the old stallions claims Filthy rich is behind it to get the insurance money or something and the cmc talk to diamond tiara, who says her dad didn't do it.

6512847 Those are secondary canon, meaning they're canon unless contradicted by the show. It still doesn't change the fact that Diamond Tiara has not had a speaking role since her redemption. The best we've gotten was a non-speaking cameo at the beginning of "It Isn't The Mane Thing About You".

6512888
They are no any kind of canon. They operate under the same rules as fanfic. Their authors attempt to align closely to canon, but fail and/or get contradicted at times. They are not "canon until proven otherwise" they are only ever canon if expressly said to be so by Meghan McCarthy, the showrunner. Otherwise they fall under the blanket "separate but interesting" statement she made.

6512847
They're basically an AU. The events of the comics, books, games, etc. are assumed to have not happened as far as the show characters are concerned.

6513125
Hm.... Back in... 2014, I think... One of Hasbro bigwigs in his interview said that comics happens "between episodes". But I'm not sure if I even able to find this interview again.

6513862
Meghan McCarthy is the Showrunner. It is her literal job to decide what is and isn't canon for MLP:FiM. She has stated that the comics are a separate thing. They're not canon. This isn't an opinion, this is a simple fact. I can provide source links if you like.

6513937
And Hasbro pretty much her employer and franchise owner. Between what Hasbro said and what Meghan said Hasbro words kinda held more weight.
For example waaaaay back then Faust stated that Trixie attended SGU. But Hasbro said that "no, she isn't", and Faust pretty much backed down.

6514004
Not pretty much. Hasbro is her employer. And Hasbro hired her to be the showrunner. Hasbro "big wigs" don't go around declaring canon, that's what they hired Megan to do.

The comics are not canon. They try to adhere to canon, but they don't determine canon. They're like fanfics. They can be jossed, and often do get jossed hard. If you personally like to build your headcanon using them, that's your business. But they aren't canon to the cartoon.

6514018
Actually they do. Lots of things at least in first season happens because Hasbro demand it. Rarity become a seamstress because Hasbro demanded that at least one core pony should be enamored with fashion. You don't see lots of stallions? Hasbro demand it. Celestia not a queen? Hasbro. Looks of Friendship Express? Hasbro. And so on.
So between Hasbro and Meghan in my opinion Hasbro wins hands down... Or would win if I could find that interview again :). Without this interview this whole dispute is meangless.