It takes some fancy coordination to make sure that the recieve address is getting the correct expected values and that none of the participants try to double spend or not sign their txn correctly. All while not assigning addresses to users (defeating the purpose). It's a bit complicated for the pl...

Incentive imbalance: Payjoin senders pay mining fees for the receiver.

Its an optional soft fork so since its optional in only some wallets it basically gets no adoption. It must be a protocol requirement for it to be standard, similar to ring-signatures on Monero.