Who decides that? SAAMI wasn't even around when the .45 Colt was invented.Only ONE of them is the official designation...
Maybe the developer of the cartridge does? In that case we should all (well, those in the know, at least) be complaining any time someone refers to the .38 Special since S&W calls their cartridge the .38 S&W Special. Should we start a letter-writing campaign to SAAMI (who just calls it the .38 Special) and the ammo companies who are also getting it wrong to make them fix it? And, of course, all the people vigorously defending the "official" name for the .45 Colt should also just as vigorously correct anyone who posts the unofficial name .38 Special on the internet.
By the way, Colt has been known to sell products with the designation Long Colt.
https://thefiringline.com/forums/showpost.php?p=5168958&postcount=38
They've even referred to some of the guns they sold as being chambered in .45 Long Colt although the link I had for that no longer works.
Finally, while one could argue that these days there's not really any need to call it the Long Colt, there certainly was at one time. When you could go into a store and there were two options if you were buying cartridges marked .45 Colt, the long ones and the short ones. (See the pictures in the following link.)
https://thefiringline.com/forums/showthread.php?t=497213
This is one of a few topics that I call "initiation" topics that tend to be used as a sort of "hazing" by the gun community.
You just can't be someone until you've been set in your place for calling a magazine a clip or for referring to the .45 Long Colt. I guess it beats some of the other methods of hazing out there...