Because it is not registered with a legal entity,
As for mixer services, basically it is impossible to get legality because serving "privacy" is not liked by the government. Then, any unregistered service will be inferred to be illegal and to contain "malicious" activity as is generally suspected.
and being registered means it's going to become centralized and there isn't going to be any privacy again; I don't know though, KYC may even be required before one can be allowed to mix,
