The RS MVP is based largely on a couple things:
1. How your team does with you
2. How your team does without you
When Steve was playing, his team was winning a lot of RS games. When he was injured, they suffered. This made him a lock for the MVP. He owes a great deal to Amare, Bell and Diaw for his success, but they owe him as well. Unlike autistic boy, he was notorious for playing through injury.
Steph won because he destroyed anyone guarding him. He shot the 3 so often and so well that he didn't even have to play defense. It helps to have another world class 3pt shooter next to you.
When KD won, Russ was injured and KD had good success running the team with Reggie Jackson at the point.
Lately it's been whomever stat pads the hardest though. It's become a gimmicky RS goal for non-contending stars. OKC took it to absurdity, the entire team was complicit.