>>543953292
Ease of access.
For the Tsukihime side Nasu has to work a lot harder to generate vampires whole cloth. Heroic spirits have legends to pull from, characters and abilities from the public domain to adopt. For about two decades now, people have postulated on their favorite legends. Even now, we may ponder on what Genghis Khan would be like as a servant, or the noble phantasms of El Cid, or the most appropriate class for Cain.
Meanwhile, have you ever for even a single second wondered who exactly is Trhvmn Ortenrosse? Some sort of ancient vampire king. Could be cool, but are you on the edge of your seat to learn more about him? Van-Fem was cool in the brief moment he appears in Strange Fake, sure, but do you think he's cool enough to support a whole story arc about his machinations? The Dark Six is worth of a whole visual novel, of that there's no doubt, but the amount of work Nasu would have to actually put forth to bring that story to life is simply a much bigger mountain than it would be to tell compared to another Holy Grail War.
Fate, in general, has a lot more room to expand and a lot less creative investiture to do so. It also meshes with the Magus Association side better, and Nasu has been filling in those gaps for years and years. Even Zelretch finally got tied back to Solomon. The whole structure of the Clock Tower was filled in with the lore of the Crypters. We got to see the Wandering Sea because the world got destroyed due to Marisbury, the winner of a holy grail war.

All in all, Fate is the ball that is easier to get rolling.