Its all about sales, they are trying to offer what they think people will buy, and buy the most. The new Winchester rifle makers I assume don't have deep pockets, not enough to experiment. Just like Wally world, Kmart, and Sears, you could get a great deal on a rifle from the big 3, in .270, 30/06 and that was it, for centerfire, and they purchased them by the boxcar load.
If they succeed, then we might see other offerings. Also, many manufacturers won't offer a caliber from their competitors, wssm versus remington short ultra mag. Its like saying their own new cartridge isn't good enough. If either caliber succeeds, you will see it offered by everybody down the line.
I personally would like to play with some of the Wssm's, but at .80 to a buck a pop for brass, not now. I would like to see a short 6.5mm that approaches winchester .264 mag power. The bullet is excellent, in BC, SD and accuracy from almost anything. But if it were available, I would still hold off, due to cost.
By the way, I'm a big 7mm08 fan, nearly bought one in the M1A, when springfield offered them, I own 3 bolt actions in that caliber. Would like to try the .260, but again, not now.