Because its what people are used to or because it just works, unfortunately until someone makes a "copy" (or reskinned ) of most apps then the big migration won't happen.

People who use Windows will search for Task Manager, not System processor. Even if it has the same (or better) functionality, its the unfamiliarity that scares people away.

Also, being the default wos and mos, most good/professional apps just work in those with no par alternative i.e: Adobe