I can't speak for everyone but I believe it is due to 2 factors. 1 is a supercharger has a parasitic drag on the engine as it is pulley driven( opposed to a turbo's exhaust driven power), and yes the horspower gains of the supercharger overcome the drag, however lets say a supercharger gives you 80 hp on 8psi you then may loose 5-10 of that on the parasitic drag, where as a turbo gaining 80hp at 8psi has no loss to deduct. the exact losses may be a little off but the principle is the same.

2. your normal supercharger (roots style) is limited in boost and is extreemly difficult to cool the compressed air as the compressor is the intake manifold. This is overcome by the vortech system, which is an awsome system and can be upgraded easily, you just need $4500 -5k to get started. also the vortech systems aren't as easy to find used and customized for your application.

yes turbos have a down side, like lag, but they are easily compensated for if you know what your doing.