I believe for most people it's a matter of ergonomics. I can do things musically on a medium or short scale bass I simply cannot do on a full scale bass. Because of the size of my hand, I cannot do one finger per fret on the first four frets of a 34 inch scale bass without causing pain. Of course, my technique suffers, as well.
I've found my perfect bass is a medium 32 inch scale with a 7.25 fretboard radius, but who is mass producing that? No one!
The truth is, there aren't enough companies building for this under-served demographic. Partly because 30 inch scale does affect tonal quality in some measure. Also, the typical Fender Mustang/Musicmaster design really isn't all it could be for various reasons I won't go into. But Fender is in the business of selling a metric ton of full scale basses, so why should they care?