For the same reason other metric cartridges took a long time to catch on here. There has been a lot of resistance against anything European/Metric among hunters in the US pretty much up until remington introduced the 7RM.
The .264wm was doomed by a handful of snotty anti magnum writers and to the lower quality barrel steel available in that era.
While at the same time the 300 and 338WM were accepted? I doubt that was the case. The 7mm Rem Mag is widely accepted as one of America's most popular cartridges and has been for decades as well as being credited for the poor market performance of the 264WM. The 6.5mm/.264 caliber itself has received little fanfare in comparison to other calibers, let alone individual cartridges. The reason the 6.5 never caught fire is it attempts to a fill a void that never needed to be filled, nor can it be filled. It really has nothing to do with ballistics, but at the same time it does.
As many are beholden to the conventions of using appropriate calibers fo rate job, the 6.5 in any variation is not appropriate for anything other than deer-sized game. Many varmint hunters prefer smaller calibers for coyotes, prairie dogs, chucks, foxes and the like. 30cal cartridges are hardly overkill for pig-sized game and above. The same is likely true for 338s. We in the U.S. tend to have the ability and inclination to use something more appropriate for the job.
The cartridge seems to be a great paper killer and has pumped new life into competitive shooting. It's really great for the sport and industry. However, the 6.5/.264 is not the be-all end-all when held against the standard bearers of the past. It's not a heavy game cartridge by any stretch, and even we on this long range forum regularly advise 6.5CM/260Rem style cartridge shooters to limit shot distance on bigger game. Are there better 1k cartridges? I would wager definitely yes. Better deer cartridges? Depends on the rifle platform and the geography to be hunted- I'd say a marlin lever gun in 30-30 would do the trick in many instances, but so would a 308.
The 6.5/.264 in any case variation- in terms of marketing especially -is an attempt to have one gun for everything. If it wasn't then people would not be repeatedly asking "Is a 6.5CM suitable for elk?, Can I kill an elk with a 260Rem?". I think that is the major rub that people are becoming aggravated with.
This post isn't as thorough as what I'd like, or what I feel provides the topic the amount of articulation that it deserves, but I think it gets to the point of why so many are tired of hearing about 6.5CM and 6.5/.264 in general. There's nothing new about it or what it does. It's really just new to North America and there are practical reasons why.