Chess is a great game with a deep complexity to it, but you can read and see everything on the board and with enough knowledge you can predict what your opponent wants to do or what his possiblities are.. You play the board and react to it Its all you basically do after applied skill and attributes because you dont need anything else. What you see is what you get.
In poker its different, you dont just play the board, you play against characters/personalities, Cards/pieces you cant see, against chips/stacksize, you play against sense and mathematical calculations but also lady luck. You even play against your own mind and because you dont know what the other person has or why hes doing what hes doing.