You're asking a reasonable question. For the general public, 6 creed does nothing that the 243 doesn't. Also people have been shooting .243 forever. Also BC is irrelevant to almost all hunters.Fair point. Lets hope it stays that way.
(As pertains to the SAUMs...I don't remember seeing a ton of 7 SAUM on the shelves, really ever, in the years immediately after the cartridge was introduced.)
The 6.5 creed was a perfect storm. It was so much better than the gold standard for long range shooting that it got adopted overnight... at a time when long range shooting was exploding in popularity. For a few decades prior the Creed's introduction, if you went to a shooting school, the ground was covered in 308 cases and everyone's truck was full of 175 gold medal match (or 118LR). By 2015, it was nothing but 6.5 creed.
On top of which, normal hunters (who shoot a few rounds per year) were seeing that a 6.5 creed kills deer as well as a 30-06. The bullet weights are similar. It doesn't seem "light" like a .243, but it doesn't kick like a 30-06. And whatever 6.5 ammo you bought at Walmart, whether it was "target" ammo or not (ELDMs), turned out to be super reliable on game. In the past, target bullets (SMKs and their ilk) were not reliable on game.
Hard to imagine that combo of circumstances happening again.