I suppose a large part of the problem is that engine technology in light aviation remains in a time warp. We still rely on primitive 1930s tech lumps designed to run on a specialist fuel produced in very small quantities of a very high octane which someone decided a long time ago would be a terriffic idea.
At the same time of this brilliant forward thinking they also decided that said fuel should have additives (Why exactly enquires young Smithy???). Four-star is long gone in motor vehicles and most engines in various vehicles will run happily and reliably with not much loss of performance on lower-octane fuels more widely and cheaply available, without additives at that.
In the 1980s motor vehicles' engines went quickly from leaded 4-Star to running happily on 95 UL, to the extent that 4-Star was phased out comfortably. Throughout the 90s diesels were developed to be smoother, quieter, better performing and infinitely more reliable than their earlier historical variants. So why hasn't aero engine technology developed in the same manner?
The technology is there, the demand is there, oil companies can't be arsed producing 100LL anymore, so what's the problem?
Smithy