Not saying anything negative as I have a 6.5CM and it's a lot of fun on paper, but the first reason for popularity is listed below. And I agree with some other posts already, therefore my second comment;
#1 reason for me is tactical scopes at an affordable price. If all we had available to the average consumer / HUNTER was the old VX III in a 3x10x50, I doubt a high BC for a 1000 yard shot would be as marketable.
2) factory twist rate. If 270 was offered / promoted in a 1:8 and was promoted as highly as the Creedmoor, I'd venture to say it would out sale the 6.5. Same thing for .257. If you could get a 1:8 25-06 on the shelf, and Berger made a 130 Classic Hunter in .257, it would be equally or more popular as a hunting round.
The new age hunter (I am guilty also) now sits in a South Mississippi deer stand with a 6.5 and a tactical scope for no more than a 150-200yd shot max that can easily be done with a standard 270 and 10 power quality scope. But, we all look like snipers doing it which is probably something to laugh about honestly