AI is terrible in games because it's literally such a hard problem. Thinking like a human, working in a group, overcoming obstacles and reacting to dynamic behaviour from the human player. This is far harder that Go which has 100x simpler rules. Additionally they had a mega ton of training data to feed into the Go model to begin with, which a game won't have as the data structures will all be different under the hood.
The reason they picked this challenge is to work on problems where the whole state of the game is not immediately accessible (games of partial information, as opposed to chess and Go). Compared to AlphaGo, it is much more similar to the situations humans encounter in daily life.
