Comment by noambrown

I think it took the community a while to come up with the right algorithms. So much of early AI research was focused on beating humans at chess and later Go. But those techniques don't directly carry over to an imperfect-information game like poker. The challenge of hidden information was kind of neglected by the AI community. This line of research really has its origins in the game theory community actually (which is why the notation is completely different from reinforcement learning).

Fortunately, these techniques now work really really well for poker. It's now quite inexpensive to make a superhuman poker bot.