Because if people use fingerprints as a password, part of getting mugged will be cutting off your fingers...
Plus, a gummy bear can fool the fingerprint scanner, and high resolution photos of your hands can copy your fingerprints.
Fingerprints should only be used as usernames.
It's not my bank account password just my wallet key. If someone is going to threaten my finger over < $100 I'd give them the money. Just looking at ways not to type a 4 digit PIN.
Ok, I'm sure the mugger will believe you. ;)
But seriously, that is why that isn't used or implemented very often. It's been proven too easy to hack.