Maybe what we are seeing now is a "mainstreamization" of the ABO/omegaverse concept? As far as I understand it, it had begun as a niche fetish based on a popular misunderstanding of animal relationships, but as more authors encounter it, it starts losing its focus on non-consensual sex and becomes just another New Weird setting to write stories in. Since the number of authors and readers who are into light and fluffy plots is much higher than that of those into animalistic rape fantasies, the number of works that focus on relationships and plot over ABO will similarly increase as more and more people discover it. It's all just a hypothesis at this point, though.
ABO finds its origin, like all of modern fanfiction itself, in Star Trek. Specifically in Kirk x Spock fics, because in startrek Vulcans go into "Pon Farr" aka heat/rut. ABO is a result of people taking that already existing dynamic and turning it into a more universal variant that could apply to almost any setting(and therefore any ship).
I have no idea why the writers of star trek chose to have Spock go into rut, but we all have to live with the consequences of that decision.
I've personally never really had any issues with encountering noncon/dubcon in any ABO I've read(at least not between the protagonists), but I'm starting to think I've just been very lucky, so I can't really say if it going mainstream has anything to do with it. Based on what I've heard about it though it does sound like straight ABO is more rapey than sapphic ABO, so that might have something to do with it too. All though that doesn't explain what others have mentioned in this thread.