NBC quite liked the first pilot overall. They only had reservations because of casting choices and felt there wasn't enough action. That's it.
Number One doesn't come across strongly only partly because of Majel Barrett. The other reason is she is not a focal point of the story.
In fairness to Majel it's possible part of what the NBC brass were reacting to was an underwritten and ill-defined character. The character wasn't prominant not just hecause of the actress, but also because of how she was written. She didn't live up to the billing just as the 1st pilot as a whole feels a bit slow and lacking in action.
I'm more interested in why the subsequent female characters were quite poor compared to the first two? Dehner isn't too bad I suppose, do we have any feedback on what the network thought about her character? Was it a change of writers that led to the change or something more subconscious once you take a woman out of trousers and stick her in a short skirt?
Now imagine the difference if Number One had been written and performed as strongly as Elizabeth Dehner.
It was never going to happen, but it has long been an interesting thought exercise to wonder what "The Cage" could have been like as a regular series story with the TOS cast. Actually we don't have to wonder much because we have seen close variations. "The Gamesters Of Triskelion" isn't that different from "The Cage" in concept. Kirk is a prisoner just as Pike was but, again, Shatner is more dynamic. Uhura, like Number One is mostly a bit part, but she's definitely more forceful and pro-active. Shaana is about equal to Vina but does seem somewhat more vigorous.
Majel was a victim of an ill-written character and by being the mistress of a shmuck who didn't know better to hide his affairs as well as having already pissed off the network during his previous production (The Lieutenant). Whether she was a good enough actress for the character might well be moot.