This is a subject that always baffles the hell out of me. You have a group of small but vocal antagonists that act as if they were required to buy a rifle chambering the cartridge and set all those they already own ablaze. The cartridge is hated like no other and it's never for valid reasons.
First, you have to know why it came about. It was not designed for deer hunting out to 300yds. In that role, it does nothing better than existing cartridges and the ignorant act as if it's supposed to replace their favorite deer blaster. It ain't. It was designed from the start as a long range precision cartridge. No, the 6.5x55 and .260 do not do everything the 6.5CM does because in order to do so, they require custom rifles, fast twist barrels and/or over-spec handloads. As JRM40 posted, it was designed to excel at long range shooting out of the box with factory ammo. Sorry but you can't change the pressure limit of the Swede or the standard twist rate of the .260 and expect a happy outcome. Plus those cartridges are already pigeon-holed (by folks who needs pigeon'holes) as sporting rounds. You especially can't alter specs when +100yr old rifles are part of the equation. So the only viable solution is to design a new cartridge that takes the good and eliminates the bad. One that can be marketed for its intended usage, without the baggage of obsolete or dead cartridges. The Swede has a significant taper to its case and it's longer than it needs to be. Short, squat cases with very little taper are more efficient. The CM fixes those issues. The CM was also designed with a sharper 30° shoulder than either the .260 or 6.5x55. These are minor but important distinctions that don't amount to a hill of beans to the average deer hunter but in the world of long range competitive shooting, it adds up.
Secondly, I have to laugh about all the "marketing" or "hype" comments. I honestly don't know where you see it. At the gun shop? From your buddies at the range? I get a whole bunch of shooting-related magazines every month and I don't ever recall sitting and staring at an ad for anything 6.5CM related. Or even seeing one, for that matter. I guess I judge it based on merit, not the ignorant opinions of others or the rantings of madmen.
This chart shows that evolution has begun leaving the 6.5CM behind but what one should notice here is that there are NO 6.5x55's or .260's listed at all. The CM has been displaced by other, specialized, new cartridges. Not grandpa's old military surplus deer blaster. The 6.5CM just so happens to also make a great sporting round with all the benefits of its predecessors and it can also take advantage of its popularity for other purposes.