I don't see why April should be singled out in any way.
Pike is the captain that could've been if the pilot had been accepted, so there's an understandable fascination to that. And through that fascination, Pike has slowly developed his own significant place in the lore, from the Menagerie through the Abrams films to the now highly popular DSC version. There is significant characterization there plus an excellent, well-liked actor.
April has none of that. He appeared once in a TAS episode about forced retirement where he was portrayed as utterly generic and had no background of any interest to speak of. He's a throwaway character just as sure as every other one-shot character in the history of the franchise, and quite frankly I could probably find dozens of other one-shot characters with far more interesting concepts and backgrounds for a potential story to dig in to.
Yes, you could write an incredible story around Robert April if you really wanted to, but it would essentially be the exact same thing as writing a totally original character, so why not do that and expand on the lore instead? If you want to tell a story about the Enterprise before Kirk then it makes far more sense to use the captain everybody knows and likes who already has a beloved actor lined up.