Well because in my simple little mind a good number of powders are OK for a cartridge. The 223 Remington and 308 Winchester are good examples to name but a few. Each cartridge can be loaded with a pretty wide range of powders. Being "OK" isn't what we always look for though. It's like there is a line between OK and well suited for.
The recent powder shortage, well component shortage in general, is a good example. It's like if we couldn't be with the powder we loved then we loved the powder we were with. In many cases this wasn't such a bad thing as many hand loaders had to settle for what they could get and many times it worked out better than using their old tried and true.
Sometimes though, not so much.
I also agree that if the guys making the bullets also happen to own a company or two that market powder their manuals will focus on their powder lines also. Just good business sense.
All in all though, some powders are better suited for some cartridges than others. When possible I prefer the well suited over the OK powder.
I'll be curious as to what Speer has to say? Please do post what you get back.
Just My Thinking on The Subject....
Ron