After all, they did get a big-name star to appear in their first pilot. Jeffrey Hunter was a well-known movie actor, and in fact the reason he didn't stay on after the pilot was that he didn't want to give up his movie career for TV.
That's not the reason.
Hunter's wife saw the pilot and decided that sci-fi was beneath him.
According to Herb Solow:
HERB: We had an option on Jeff Hunter for a series, but not for another pilot... the lights came up, Jeff Hunter's wife [actress Sandy Bartlett] gave us our answer: "This is not the kind of show Jeff wants to do, and besides, it wouldn't be good for his career. Jeff Hunter is a movie star."
--Inside Star Trek (hardcover), p. 63
I don't think that exactly says "sci-fi was beneath him". It's possible Hunter did the pilot because it was a paycheck and he never expected it to go to series.
Elsewhere in the book, and I'm having trouble finding it, I think there's reference to trying to get Hunter to come back and at least shoot some addition footage so that The Cage could be released theatrically, perhaps overseas...but I'm having no long finding that passage, so don't hold me to that one.