Lack of support, in a word.
Lots of people slate the 6.5CM (and other Hornady designs) as just being marketing but that’s not really fair. Marketing is certainly some of it but they also make sure the logistics are in place for a cartridge to be a success (and naturally, having poured that much effort into it, they market it so it can be a commercial success as that’s good business).
It’s also well thought out, they spotted a gap in the market for high BC bullets and made sure the SAAMI spec would allow these to work.
The Creed was released alongside rifles that could fire it and a line of quality, and available, ammo. Hornady have done similar with the everything from the HMR (although quality may be debatable on the ammo side) to the newer PRCs.
Other manufacturers just don’t seem to get this, they may come up with a good idea but it’s not supported or properly thought out, e.g. using slow twists, not having an ammo manufacturer to make enough ammo, lack of rifles and so on.
Could the 260 have occupied the niche now filled by the Creed? Almost certainly, but it didn’t and now the Creed does. Outside of hand loaders who like something less mainstream, there’s simply no reason for someone to buy one over a Creed.