I'm sure they looked at the finished pilot and decided that certain actors made an impression and some didn't. Andrea Dromm was really just there. She was sweet and pretty, but Grace Lee Whitney was stunning, almost burlesque like and her personality resonated the moment she stepped on screen.
Dromm would need more episodes to build on her character. In WNMHGB, she's clearly meant to be sort of green and not exactly prepared for all of the mysterious things headed their way.
Paul Fix was a good actor, but not outstanding. I like him as a performer, but there's no spark. He's too old for what became a breakout role.
Yes, Fix is good in other roles, but he was far too old to be the kind of contemporary Kirk needed on the human side.
Lloyd Haynes was also a good actor, but Nichelle had a glow about her
But Haynes was a superior performer. It would have been fascinating to see him make the series cut with what he could deliver.
Honestly, the one actor who really sold the episode? Shatner. His magnetic performance blows everyone off the screen. You could have recast everyone else and I would have watched it every week. Even if they dropped Nimoy - whom I loved but Shatner was the guy I gravitated towards.
Well, aside from all of the numerous great elements of the second pilot, Shatner--as you say--sold it all. He made that role his own from his first shot, and explored a number of emotional states making him a compelling figure. Almost inarguably Roddenberry's greatest decision for the Star Trek project/concept.