I would suspect it's easier to make a profit or more of a profit with selling 3 rounds, vs. expanding the lineup to 5 rounds, etc...

forcing customers to pick a .260 or 7mm helps profits vs. if those 2 groups were split up to a .260, 270, 7mm.

I think often times it comes to what's popular in those lineups, as in the 270win was hugely popular, so why try to compete with it with a 270-08 and steal some of its thunder.

btw comparing the 270 and 7mm, I would say whats the SD difference from a 130gr 270, and a 140gr 7mm?