This is my opinion, but I've always thought the main draw of the Omegaverse is to impregnate men. And the only way to do that is through alphaxomega. That said, alpha on alpha dynamics are also fun. So fun to the point mf have invented enigmas. JUST to impregnate alpha's
The dynamic of alpha on beta is...well....just normal BL. Depending on the setting, betas can't even perceive pheromones, so a setting of alpha on beta could just as easily be rich master x normal student.
Mind you, I'm not against the pairing, but I feel like a lot of authors start their stories without the will to commit to a normal BL dynamic (no pheromones, limited or no ruts, no babies).
I think it might just be easier for them to be like "oh look, he turned into an omega! Or an enigma!" And partake in what the Omegaverse has to offer.

BRUUUUUHHHH and here I thought I had hit the jackpot. Why is it that everytime there’s an Alpha x Beta story, the damn Beta turns into an Omega? Alpha x Omega plots are getting stale pretty fast for me. A Beta x Alpha story seems so damn interesting but not one singular writer actually commits to it!!!! Fuck me dude…