I mean, there are stories about every other equine pokemon on this site, so why not everyone's favorite long neck pokemon?

Also, there needs to be more keldeo stories too!

Here's one for you:

http://www.fimfiction.net/story/242914/pokemon-battles-are-friendship

It has both Girafarig and Keldeo in it. Whether or not it's actual quality work is up to you to decide though.

As for your question, my guess would be that Girafarig is not the most popular Pokemon ever. It's not hated, but it's just a little bit more obscure and doesn't come close to the heavyweights like Sceptile or Lucario in the popularity contest. Not to mention that it's not exactly equine even if it comes close in terms of shape and design.

Keldeo on the other hand, I thought he was actually moderately common in Fimfiction, possibly even overused. Unless I'm mistaken of course.