The advantage of the 6.5CM over the 6.5x55 (amongst other things) is the higher operating pressure. 20" 6.5CM easily produces over 2700fps with 140gr. Even with a few hundred bar spare. 6.5x55 shooters often seem to run the cartridge on the ragged edge. It does have a much lower max pressure and brass is made to that spec. There is no +P 6.5x55 brass that I know of.
I think many were forced to use the 6.5x55 in northern countries because it was the army issue, a competition standard and maybe ammo was kept cheap in certain areas. We only see how much it was disliked after introduction of the 6.5CM. I thought one big reason would be the AICS magazines that seem to push people from 55 to CM but now we have medium AICS magazines and it doesn't help x55 out of the grave either. MDT have a solution for AICS type magazines and mag systems in 284 win length that fit into a Tikka CTR stock and could take 6.5x55 or 284 Win..... we have not had one taker yet for 6.5x55. Similar with Rem clones types, Medium actions are so slow selling that magazine system manufacturers and custom action manufacturers are talking about dumping them. Rather run the mediums on long actions and make a mag system with medium magazine on long action floor plate.
edi