Problem is, PayPal et al work in government-backed currencies. You can expect that when someone pays you $1 over PayPal, you'll get $1 worth from it (regardless of how many Euros you'd get for that dollar). However, with Bitcoins you'd be running into the issue of a double exchange. First you'd have to trade your dollars for Bitcoins, transfer the Bitcoins, then trade them back into dollars. You'll always get 1BC when someone sends you 1BC, but from the time that BC is bought to the time it's converted back to dollars, the currency could be worth a lot more (or a lot less).

Dollars are not inherently better, but at least I can immediately spend one and get a soda from the gas station. Until Bitcoin reaches that point, people will be sending dollars over Paypal.