I'm new to /jp/ so I can't be sure, but I assume it's because it's one of the "big names" so it has probably been discussed to death already.
It saddens me because it's my favorite out of the few I've read. I'll admit I'm completely new to the medium but to me it seemed miles ahead of Sharin No Kuni, F/SN, Saya No Uta or basically anything else I've read. It just striked me as more mature, and something I could actually imagine showing to my friends, if I had friends.
I compensate by reading old threads about it on easymodo and trying to get a discussion going with something a little more original than "so I've just finished tsukihime, discuss".
See
>>7285668.