Hi,
It is because concrete doesn't transmit tactile energy (vibrations) in the same way that a raised wood floor does. There are advantages and disadvantages, though, to both approaches. Wood conveys much more tactile energy, through a chair or sofa, than concrete does. But, that energy transfer can also cause walls, which connect to the wood floor, to vibrate more, and other things in the room to rattle. Being on a thick concrete slab does, however, require more subwoofage, frequently in the form of ported subs, to obtain the same tactile sensations that a person on a raised wood floor will experience with smaller and less powerful sealed subs.
Regards,
Mike