People are often more careful not to fall for other poker players bluffing than slow playing. In other words, people think their opponents are more likely to oversell than to undersell their cards. While both are effective strategies for luring bad bets, they rely on different psychologies.
Having the courage to call someone's bluff requires confidence to bet that you have better cards. On the other hand, when betting against a slow player, you need to be humble to the fact that there may be a better hand on the table.
Slow playing is particularly effective against players who can be over-confident. Because slow playing requires a strong hand, people tend to discount the likelihood that anyone would actually employ this strategy. Try to remember that if someone is betting weakly, there's a chance that they're playing you against yourself.