RE: Why did the .260 Remington not take off as a standard caliber?
I think the issue is the existence of the .25-06 and .270 as they shoot "flatter" to name one feature. These calibers can do everything or more than the .260 and are simply more popular for whatever reason (the .270 has a lot of history behind it and famous characters that used it [Jack O'Connor] and developed for it - [Weatherby] come to mind).
Very similar, in my opinion the .280 is a great chambering but almost no one makes rifles for it, don't know why...