Fair enough bloble, I've been stuck with just fate fans here for awhile now so that statement was more of habit then accuracy. But it is mostly Fate in the game so far you have to admit.
I'd say it's because Tsukihime has too much bullshit. Out of all the characters in it, only Ciel or Akiha are viable as good guys, and only SHIKI, Roa, and Nero Chaos are okay as villains, and they range from 'kinda stronger than a normal human' to 'can almost tank Excalibur'. So no easy power balancing there.
Shiki and Nanaya, forget it. If someone brings them in, someone else is going to rage the moment their character gets permanently injured in a fight and they can't bullshit a regeneration. So with these two it's way too easy to do too much damage or go crazy. And Arc... well, bringing her in unnerfed would put her square at the top of the food chain. Not a single one of the current characters can even touch her, so you'd need 30% Arc or a similarly weakened version.
Oh, and Shiki's not as dumb and suicidal as Shirou and co. Also another reason why he isn't as popular and thus not roleplayed as often.