Probably because we restrict their crawlers. But this is an excellent side effect, because the last thing I want is traffic from Google searches.
It might be a bad idea for some reason I cannot conceive of right now.
Other is make it difficult to access by a random, specific criteria, such as technical hability. Yes, I am no tech hacker, never used Tor, but I try to be part of this community as valuable member. I think community is about aligning interests, not an exclusivist club.