Could be that - to an extent - this smells like a really thankless job. If you’re Shatner and have alimonies to take care of, there’s money to be made, sure, but if you’re an uncredited (even if recurring) extra, you’re barely a curiosity. Might not be worth the hike if you’re not gung-ho on cashing in on your acting gig that never really worked out.
So… I could see someone making this decision while fully aware of what Star Trek is.