It's not that they won't, it's that they legally cannot. They don't have the Ultimate license or the older games.
Thor is fantastic, one of the best Thors they've made. He really impressed me, right up there with Wave 44 Thor. Wesker, MODOK, and Akuma just make the wave. Great way to end the line.
But Hsein-Ko and Tron Bonne are in regular MvC3.