I think it's that Brunson has been the leader and the best player on the team, probably ever since he got signed to the Knicks. He pretty much turned the franchise around. And usually, that team leader wins finals MVP.

It's a subjective reward voted by a panel of "experts" anyways, so it really just depends on how the panels' subjective reasoning thinks when they cast their vote. These things can change on a whim and is hard to predict, especially when you have more than one player on the winning team playing exceptionally.