I suspect two other reasons the .260 didn't sell like Remington hoped were the .25-06 and 7mm-08, which had already been around for a while. Hunters who wanted more power than the .243 could buy a .25-06 if they wanted more zap with lighter-weight bullets, and hunters who wanted more bullet weight could buy a 7-08. As a result the .260 seemed to many a round that was going over already plowed ground.
Of course, that was back before so many hunters had problems running long-action bolts, or were aware of the incredibly enormous advantages of 6.5mm bullets.
I may be off base..... let me restate that, let's assume I'm off base, but I think that there is a simple reason wy the 6.5 Creedmoore is as successful as it is, and that is it's lower level of recoil. Perhaps especially recoil velocity.
Hornady presented the cartridge as accurate and long range, which is essentially what the magnums promised starting with Weatherby: long range shooting. In the real world, though, even super macho, tough as nails, uber-studs don't like 300 Magnum level recoil. Or 7mm Rem Magnum level recoil. Or, really, even 308 level recoil. We started shooting AR's and realized we liked shooting, just not the big boomers.
I never minded the recoil of my 358 or my Whelen. The 358 Norma and the 7mm Rem I found both about equally obnoxious, and I think that's because of recoil velocity of the rifles in those rounds.
The 6.5 CM gives people that fancy "long range" shooting an excuse to get away from the big boomers and shoot something they don't hate pulling the trigger. The 260 never gave them the cover of "long range accuracy" and "latest greatest" like the 6.5 CM does.
