Market forces, pure and simple. Most hunters are "ballisticly challanged", and don't have a very firm grasp on riflery in general, let alone a clue on performance. To them, if it doesn't kick like hell, cost $40 per box, and rattle their teeth, it "just ain't powerful enough". Lots and lots of good cartridges around that just hang on...they are not discontinued, but they are not popular, either.

I like the 6.5x55, partly because I have five of them. It can do anything the .260 can, so there really isn't any practical difference.