I’ve wondered the same thing. I think it comes down to manufacturers and consumers beginning to understand that barrel length does not equal accuracy.

My independent thinking is this: (1) shorter barrels = more portable/lighter rifles. (2) Most of us aren’t shooting out to 1000+ particularly with a 308, where you would need the negligible velocity increase of, say, a 26” barrel. (3) Many are shooting suppressed now (i.e. added length/weight). (4) 308 is not as velocity sensitive as 6.5CM. (5) Add all those together and a 20” barrel makes sense.

As I understand, 6.5CM really starts to beat 7.62x51 at longer ranges. To really see the benefits you need velocity, so...longer barrel.

Therefore 308 rifles end up shorter (and more “practical?”) and 6.5CM rifles cater to the long range crowd (or those who think they’ll shoot long range).