That's because the WSMs came out first (I think) and because the WSMs are a little faster.
The really good thing about the SAUMs is that they can truly fit in a small action.
I think that, in the WSM line, the .300WSM is definately here to stay, and the .270 WSM is in second place. I don't expect the 7WSM to last too terribly long.
Zachary