Because very few bizjets are big enough to accommodate underwing engines.
Aft mounted engines require heavy structural adaption of the fuselage, a backwards c of g with associated limited travel, and large horizontal stabilizers to provide sufficient tail volume for aerodynamical stability. And in many cases, it requires a T-tail, again a heavy structure. Furthermore, wing-mounted engines provide some wing bending relief. The advantages -clean aerodynamic and (somewhat) smaller vertical tail- are outweighed by the penalties.