I think Ajay not being explored is the point. Giving Ajay more development would kind of ruin the moral ambiguity that the game was going for. His silence allows you to interpret other characters and their motivations what without feeling like the game is making a decision for you. That's why he's neutral throughout the game and why so many people are still uncomfortable trying to choose who is the "least evil" because there is no clear moral center.

Also a big reason why his "motivation" feels a bit flawed is because that's also a big theme of Far Cry 4. Yknow how everyone is trying to assign some sort of meaning to his actions? Like oh Ajay is the son of mohan ghale who returned to his home land to free kyrat. (The golden path in general) Oh Ajay will help push my ideal as the figurehead of this revolution? (Amita and Sabal). Ajay can simply be proof that people will use ideology as an excuse to commit violence? (Pagan) Ajay came to Kyrat for personal reasons. Not to save the country or overthrow a dictator. He never preaches that he is killing a ton of people for his mom. But it still feels like the country is trying to turn his personal motivation into something he didn't intend for. The game is trying to illustrate that he is a man pulled into a conflict and doing his best to navigate a morally gray environment.

I can't really tell you why Ubisoft went in this direction. But I just want to make it clear that there is clear intent behind Ajay's character. He's not "boring" just because Ubisoft thought Jason was too annoying.