One reason might be that it's a lot easier to rig up a custom turbo setup than it is to rig up a custom supercharger setup. You can turbo pretty much anything if you have access to a machine shop and have the fabrication skill. The supercharger kits usually have to be custom made for a particular car, and they're usually only offered by one or two major manufacturers. For cars that kits are offered for, they're not uncommon. I've seen many jackson racing supercharged civic si's and integras on the pages of various import magazines, probably more of them than turbo'd si's and integras.