Complex sentence: Unlike MP/M, it never became popular as a single-user but multitasking OS, partly because of license costs and partly because of the requirement for special device drivers--unlike multitasking, but single user, DOS additions such as Quarterdeck's DESQview.
Split into simpler sentences: Unlike MP/M, it never became popular for single-user but multitasking use. It was priced as a multi-user operating system, of course, and required special device drivers, unlike single-user multitasking DOS additions such as Quarterdeck's DESQview.