The sad fact is, GR was a sleaze bag in this regard. He cast TOS's female regular roles, all three of them, with women he was having sex with or getting sexual favors from, and it showed. STAR TREK suffered for this.
The really good, talented, and let's face it, better-looking actresses were getting work on other shows without trading favors for it. STAR TREK was good, but if GR had been a better man, the show could have been so much better.
Director Ralph Senensky alludes in his blog to a problem the show was having, with desirable actresses refusing to come in for guest roles on STAR TREK. Guess why.