Because otherwise ordinary people will not have access to BTC and you can't achieve adoption without them.

A Bank has an infrastructure that could make BTC virtually omni-present in the physical world. Any other non-gov institution, would have a hard time setting up an infrastructure in large scale. A bank has also the knowledge to protect herself from bitcoin itself, because (supposedly) understands the financial (and social) ramifications of every choice made AND should be held accountable if things get out of hand.

If UBS (as in the example above) starts accepting BTC transactions, it means that thousands of hundreds of average Joes get instant access to the system. Otherwise, it's just you me and a couple of bleeding edge technophiles.