Because they've analyzed the data and determined that those studios give a poor return on investment. As simple as that.
Lionhead, especially, has been a money sink. Their last game was so poor that it was cancelled even after $75M had been poured into it. Certainly, an argument can be made that Microsoft was at least partially responsible for that mess but that doesn't change the bottom line.
Evolution is tougher since a lot of their problems have been bad luck. They were a good studio overall.
Under that is the fact that Microsoft and Sony are finding that supporting specific games from third-party developers is more cost efficient. They get to pick and choose which IP they back and, if things don't work out, they can just walk away.
