Protagonists are not usually the most developed characters, both in this series, and video games in general
Sure, I guess it’s kinda funny to joke about how “Phoenix steals the spotlight from Apollo in Ace Attorney 4,” but seriously. Video game protagonists are not defined by narrative significance or character development. They are defined by how much you play as them. In many games, the protagonist is simply your window into the world; very often protagonists have no arc and have fairly straightforward personalities, assuming they even talk at all.
In their debut games, Phoenix, Apollo, and Ryunosuke are all written to be a relatable everyman/straightforward hero. They’re regular guys living in a mad world, and that’s what helps the player identify with them.
Say what you want about Phoenix in AA4, but it’s definitely his most interesting portrayal. And it’s precisely because he’s not the protagonist, so they don’t have to write him like one.
In AA5, Apollo is an edgy loner who accuses his coworker of murdering his best friend. Why? Because he’s not the protagonist, which means he can afford to make major mistakes. In AA6, Apollo has a crazy new backstory and development revolving around his revolutionary adoptive dad. Why? Because he’s the co-protagonist and the role of “normal guy in a mad world” is played by the other protagonist.
Edgeworth is a bit of a different case. He’s the only protagonist who wasn’t initially written to be a protagonist. By the time of his first leading role, he was already firmly established as a rich, smug, hyper-intelligent, comically serious, jerkish, nerdy, morally-gray crime-solving machine with low emotional intelligence, a tragic backstory, a foster sibling who constantly whips him, you get the point. Basically, he’s not exactly the most normal person. So the Investigations games couldn’t really write him as a normal protagonist.
tldr: Athena would make an interesting protagonist because she was originally written to be the third in a trio of lawyers, and because of this, she‘s far from the most normal person. If they introduced a sole new protagonist at this point it would probably just be another flavor of what we’ve already seen.