ABP,
I have been flying Mogas for more than 25 years. Used properly, it is an excellent aviation fuel. That said, there are many installations where it just does not work well, or with an adequate margin of safety. Without going into a long dissertation on this, in general, the compatibility of the gasoline with the engine itself (which is mostly the anti knock "octane" properties of the gasoline) are only one of several important elements.
The ability of the aircraft fuel system to safely deliver the fuel is every bit as important. This centers on the considerations of "vapour lock" among other factors. Therefore, you can have a Mogas approved engine in an aircraft which cannot deliver the Mogas to the engine safely. I did a lot of Mogas testing in the late 1980's, and have first hand experience with these factors. This is why a "Mogas STC" is a combination of an engine and ariframe STC for the subject aircraft - it's not just the engine.
Though new genertion airframe fuel systems can be designed to be Mogas compatible, existing airframe systems may not be compatible. Obviously, the cost to retrofit and approve these systems could be very high, and there is little incentive, as few owners seem eager to spend tens of thousands to retrofit their aircraft to a newer fuel system. If they were en mass, we'd be seeing leaps and bounds of progress - we're not.
Then add on top of that, the reality that Mogas is being changed to contain "oxygenates" which can wreak havoc with some fuel system materials. So, even if you get the fuel system working with gasoline, you have to account for the non gasoline elements of the "Mogas", and you're starting all over with the approval.
Diesel engines and Jet fuel are the way of the future for many GA aircraft. All it takes are the large number of aircraft owners ready to make the investment in these aircraft, to provide the market incentive to the engine manufacturers to overcome the development hiccups.