Mule Deer has addressed this question/point in a general sense many times.
Money! But not in a bad way, in a capitalistic way. Stuff that generates revenue sticks around. Unlike the good members here, not everyone who is a hunter is a gun slut. In fact the best hunters I have met consider their rifles/shotguns as tools. "Wipe down the crescent wrench and put it up until needed". These guys get by with one or two rifles for a life time and generally use boring old cartridges that just work and are available where ammo is sold. I own a 280 and can, with confidence tell you the one thing that it tangibly does that a 270 does not, which is cost 50% more per box of ammo!
I felt that during the short and shorter craze the manufacturers through a whole bunch of stuff at the wall to see what would stick. Whatever people buy will stick around. Some of the cartridges that didn't stick may well have been superior but not enough so for people to wander from what already works and is readily available.
That's accurate but often doesn't have much correlation to the merits of a cartridge. Perfect example is .260 Rem vs 6.5 Creed. The .260 had a 10 year headstart, but Remington botched it about as much as imaginable while Hornady hit a homerun 10 years later with a virtually identical cartridge. Would imagine some Remington staff were kicking themselves when they saw what could have been possible with .260 rifle and ammo sales if they had done a halfway competent job with the .260 Rem. If the .260 was any lesser cartridge, it would have already been forgotten, but it's hanging on because with a 1:8" twist barrel a handloader can get similar results to what people can get out of the box with factory rifles and ammo with the 6.5CM.
I often wonder what shape the .280 would be in if Remington hadn't botched the PR for it multiple times...
