Poker is about hiding what you have in your hand in relation to what you assume others have.

A psychic's facial expressions would be harder to hide as they would so accurately predict the outcome of each hand.

Additionally, until the last card (the river) has been placed down, it is unlikely that the psychic would ever truly know whether or not they'd win the hand and even at such a stage they'd be easy to read and the opponents would fold seeing that it wasn't a bluff at all.

A psychic is focusing so hard on reading and understanding the minds of others and the cards which they hold that they will lose track of maintaining their own poker-face and thus be entirely readable the entire game.

If the opponents were to then discover the psychic's power (and hypothetically not disqualify the psychic from the game since they think they are just an amazing guesser), all they'd have to do is read the psychic's expression slips at every person's betting chance to know how good or bad that person's hand was.

It is humanly to much stress on the brain to simultaneously maintain a poker-face whilst completely getting into the minds of another and discovering what an amazing or shit hand they have.