The lack of popularity of the 7WSM has nothing to do with ballistic performance. It's so close to most 7mm Rem Mags that it's basically a toss up and the 270WSM is pretty much half way from the standard 270 Win to the 270 Wtby.....guess the market decided there was more of a gap that needed filling in between those two....and Winchester really pushes thier new 270.....
Some day I hope to locate a Kimber Montana in 7WSM...the only WSM cartridge I'd care to own.
America loves the 30 caliber in many cartridges, so it's no surprise the 300WSM is more popular than the 7WSM...consider how many relatively popular 30 calibers there are,..30-30, 308 Win, 30-06, 300 Win Mag, 300 Wtby,...the list goes on a ways....7mm bores?...not as popular as 30 cals...7-08 and 280 Rems are not as popular as thier 30 caliber conterparts, 7mm Rem Mag is easily the most popular 7mm...and the larger cased 7's, like the STW and the RUM aren't that popular....popularity has nothing to do with ballistic performance...look at the popularity of the 30-30