Both cartridges are constrained by the overall length of the action they are run through, typically a short action (magazine length comes in to play here).
Your first statement is off base.
If you are seating the exact same bullet to the exact same overall length in both cartridges, there will be more bullet inside the case on the .260. Always.
You can seat heavier bullets in the CM without fear of the base of the bullet intruding past the neck/shoulder juncture.
The .260 does have a very slight edge in case capacity, and a very slight edge in velocity, but the 6.5 is a more efficient case. It has been proven time and again that a shorter, fatter powder column is more efficient, that is exactly what the 6.5 gives you.
I don't know what action cycling time has to do with the price of tea in china, it is the same action, same action length, you pull the bolt back till it hits the stop and run it forward. Case length has nothing to do with it.
In all honesty, there isn't a thimble full of difference between the two. If you have .260 stuff and don't mind reloading, then it is a fine one to use, and brass is extremely simple and plentiful.
The CM was designed specifically as an across the course competition cartridge, it just happens to excel at many other things.
The reason for the popularity of the CM comes down to the availability and price of match grade factory ammunition and factory made tactical/precision rifles.
Just because remington fucked the pooch in marketing and support for your pet cartridge, don't get your panties knotted up because Hornady did it right!