I think there are several factors. But the main one is that Rifle shooters today are more concerned about long range accuracy and target shooting than hunting. While the 6.5 Creedmoor may be a good and more than enough for deer, it doesn't offer any major advantage over other popular cartridges at hunting ranges. But it was designed for accuracy by Hornady engineers with help from bench rest shooters and is based on bench rest specialty ammo. It is designed to use long target bullets making it a viable long range target round. It is a success.
One thing I find amusing is how many people used to sneer at simple old cup-and-core bullets, yet with the rise in popularity of PRS and long range precision shooting, the 6.5 Creedmoor, etc, a lot of the same people now suddenly love these sleek cup-and-core bullets like the Hornady ELDs. Sometimes you'll see them run numbers and point out how much more energy is retained over dumpier options. So either the fancy bonded, partitioned and mono "premium" bullets aren't necessary for most hunting people do, or a cup-and-core impacting two or three hundred feet per second faster beats a "premium" bullet that hits a bit slower.