I'm not qualified to give a comprehensive answer, but I would have to imagine that it partly has to do with the fact that poker is a game of partial information

I'm not qualified to give a comprehensive answer, but I would have to imagine that it partly has to do with the fact that poker is a game of partial information. In Go or Chess the current state of the game is known. In poker, you don't know what cards your opponent hold, so you have to assign probabilities based on previous actions, and simulate many different scenarios. Another part has to do with the fact that both you and your opponent have surprisingly many legal "moves" at each turn. because not only must you decide to fold, call or bet, but if you bet, you also have to decide how much.