Banks serve three purposes, the first two of which are unnecessary with Bitcoin:

1. A clearinghouse for transactions

2. Safe storage place for money

3. Connecting lenders with borrowers

The fact that no one needs a Bitcoin bank for purposes 1 and 2 means that any lending entities/banks will have much more difficulty getting people to hold bank credit as payment in lieu of the actual money itself (a Bitcoin transaction on the blockchain)

Fractional reserve lending works for dollars because people tend to keep their money in banks for purposes 1 and 2, and the banks are able to tell people that they have more dollars in their account than actually exist in the bank's posession. It will be far more difficult for fractional reserve lending to take off with Bitcoin because nobody is going to want to have someone else hold their Bitcoin for them, and thus no bank will be able to tell them they have more Bitcoins than actually exist on the blockchain.