Why did early artificial intelligence systems do so well with board games?

Because board games give the system unique insight into human behavior, early systems could learn and mimic the same behavior.

Because board games are inherently chaotic, the system had a lot of opportunities to crunch new data.

Because even with their limiting processing power, early systems thrived in a world of simple rules and pattern matching.

Because computer scientists could do a good job programming all the rules into the game that the system would understand.