There's nothing magic about it... The .260 Rem is just a very efficient cartridge that is soft-shooting and is well-suited for lightweight hunting rifles that handle very well. My Kimber 84M Select, for example.
There ain't a deer out there that will live because it was shot (given equal shot placement) with a .260 Rem instead of a .25/06 Rem, .30/06 Spfd, 7mm Rem Mag, 7mm-08 Rem, .270 Win, the list goes on... The opposite is also true.
Most everyone who doesn't like the 6.5mm's probably hasn't used one hunting. Those that have will likely never change. It's all a matter of preferance... I prefer the .260 Rem for it's long, pencil-style bullets that outperform their heavier .308" counterparts in SD and penetration (given equal parent casings). I also like the fact that the rifle I use weighs in at around 6.5-6.8 lbs.