Because BTC is what they take. The coordination is more important than the tech. Granted, this is only because I don't patronize the super illegal Go To Jail Go Directly To Jail Do Not Pass Go type of merchants, but the point stands.
If you just need unfreezable funds and pseudonymity, not total cryptographically secured anonymity, you'll use Bitcoin instead of a dozen better methods (which are all crypto anyway, so wouldn't really count as a point against crypto, just BTC) because everyone else is using it.