Definitely not just milking it - they certainly could have drawn out the first game for two seasons of they really wanted to do that, but they've made it clear they want to stick to just the games and tell a focused story.
Thr second game is bigger in scope, and is at least twice the length of the first game, so two seasons definitely makes sense or it would feel rushed. A longer single season is an option too, for sure, but I get the impression that's not HBO's style.