The market for a true "base" scanner dissappeared in the late 1980's. Since them, the market has shown indeed that today's scannist looks for an simple package with as much multifunctionality as possible. Portability is a huge thing now in all electronics as much as possible. With scanners, it became much simpler to produce just two versions of a product than three.. Almost every base/mobile unit has a little sister if you will handheld. They are both almost identical in functionality, but the b/m has more powerful speaker etc...
To creat a third version of the exact same scanner is not cost effective, and the market has not shown an interest in the idea. Now in the CB, HAM, and commercial telecommunications markets you still find all three. Though in most cases the base radio has evolved into a console based unit for commercial/public safety use.