Why don't most Bitcoin exchanges like Mt. Gox accept debit cards?
Main reasons :

1) Bitcoin is anonymous, free and irreversible. Credit and debit card transactions have to be reversible, Paypal transactions are reversible. So such a system would be a gold mine for card thieves, money launderers, scamers, etc. A relatively small website cannot face the risks.

2) If you're dealing with credit cards you have to follow some regulations, pay charges to the card emitter network, etc.