The problem with slow playing is that sometimes you have to fold on the river if you think your slow playing line has allowed your opponent to improve to beat you. Most players do not have the post flop skills to make these lay downs. Sometimes the pot size dictates a call in these spots. In an unraised pot, slow playing hands such as 2 pair (relativly strong value) becomes counter intuitive. You want more chips in the pot, so that you can win them. Since the pot is so small, due to it being unraised, slow rollling will only get minimal value from worste hands, while allowing your opponent to catch up and beat you for cheap by the river.