Because people actually use bitcoin. It’s heavily used for transactions for gray market activities (its very popular for online gambling transactions).
Why is it so popular among online gambling transactions?
Afaik to be able to withdraw money from a gambling site you will go through a screening (id, address verification) so anonimity is broken anyway.
US banks aren't allowed to process transfers directly from gambling sites to players. So poker sites etc use Bitcoin as an intermediary step.
Some sites do not require full blown identification for gambling when you use bitcoin. Coupled with the fact that it is a super convenient way to move large sums of money in and out (without waiting for wire transfer delays and whatnot), it’s a bit of a no brainer, at least in terms of convenience.