They aren't more widely supported because they are more expensive and more complicated to manufacture with a higher potential for more stuff to go wrong.
Until the engine that powers a PHEV is nearly drop-in ready for a replacement (for example, going to your local auto parts store and buying a replacement like a battery) then companies need to have service technicians and production lines to support these "engines" (they're fancy generators at this point).
However, that would also require automakers to standardize to some degree or potentially cannibalize their own business.
We've already seen this with batteries/panels in the consumer space in regards to solar. I can buy whatever packs of cells I want, and as long as the voltages match up, I can mix and match to my hearts content. If I can only get service for my Jeep PHEV from Jeep because the drivetrain is a bespoke black box and parts are impossible to get, then we'll keep seeing customers continuing to opt for traditional gas vehicles or full EVs. PHEV is just too complicated to support long term (imo).
If 90% of your trips can be covered by a normal EV, then I would make the argument that you should buy one of those (secondhand even!) and then rent a vehicle for the instances where you need AWD. The fuel and tax savings should likely make up for it in the long run. For that one year that you don't go skiing in the mountains, then you're coming out on top financially!