Probably demand. Huge corporations have access to enough data to make your head spin. EB is probably pretty good at predicting which artists would likely do well and which ones wouldn't.
Plus, there's always a chance that other artists were approached and simply did not want an endorsement deal. I've actually seen that with a guitar player I know. He wanted to be able to play whatever he wanted. And the money wasn't huge anyway. It's not as though Miss Dorsey would profit six figures for her signature bass. Neither would the others. Even Lukather (who probably sells more guitars than any other endorser other than EVH) isn't getting rich off of guitars.
(By the way, I'm just glad to see her name mentioned. She's probably a little overlooked in my view. And she's a ridiculously singer AND bassist.)
In short, it's WAY more complex than simply asking the question "Does this artist deserve a signature model?". Of course, everyone you mentioned does. It's just not that simple.