10 minutes is part of the pacing of the network.  A new block is found about every ten minutes.  The difficultly setting is there to keep the pace at 10 minutes.  This way there is a predictabl rate of new coins added into the system. 

For more information:  https://en.bitcoin.it/wiki/FAQ#Why_do_I_have_to_wait_10_minutes_before_I_can_spend_money_I_received.3F

That said, with the increasing network hash rate the frequency of blocks have been slightly more than 6 an hour, meaning faster confirmation times:
http://blockchain.info/charts/avg-confirmation-time

The short answer it is a by product of how the network was configured from the get go.