A combination of needing a heavyweight VM separately from the browser and being either interpreted or having to wait for a slow JIT. Remember Javascript was sl

A combination of needing a heavyweight VM separately from the browser and being either interpreted or having to wait for a slow JIT. Remember Javascript was slow as ass too until the late 2000's when browsers were implementing efficient JS engines. If web pages would've been build like common JPAs they would've been as bad as applets.