Actually, it's because a river bet in limit hold 'em will usually be fairly small in comparison to the pot. In no-limit, you can bet the pot, or more, and effectively offer the person who can decide to fold or call odds of 2:1. That way, he needs to have the best hand at least one time out of three in order to be profitable. For limit, the pot odds
will invariably be better than that, so the odds you're getting will be better, and you would be wrong to fold.
If I know that there's exactly a 10% chance that you're bluffing the river, and the river is 11 big bets, I must call you every time. My winning percentage at showdown will be 10% (very low) but I will be making money. The larger the pot, the less often we need to be right. Having 100% at this statistic means that we're folding winning hands WAY too often. Way, way, way too often.