There is a market for securing transactions.  Markets consist of both consumers (bitcoin users) and suppliers (miners).  Miners could have decided to go to 0.8, but it's not clear to me that 0.7 still wouldn't have won out in the long run.  The consumers could have decided to stay on 0.7 and any miners still mining on 0.7 would have still been providing a transaction security service to those users.  If that service was adequate (blocks continued to be found at a reasonable pace), it's quite easy to imagine that as the 0.7 chain continued on, miners that had previously switched to 0.8 would realize that they weren't servicing a large population of users...they could have then made the rational decision to switch back to 0.7.  The 0.7 chain would slowly regain momentum, overtaking 0.8 and ultimately providing transaction security to the entire community of 0.7 and 0.8 users.  Those miners that had continued on the 0.8 fork would lose all their mining revenues.

Conversely, consumers, seeking the most secure service and seeing the mining community transitioning to 0.8 might have made the rational decision to incur the work of upgrading to 0.8.

The decision that miners made was to serve the largest pool of consumers of their services.  Targeting the largest market of users with your mining capacity is probably the best policy (and would likely ensure that you, as a miner, are on the block chain most likely to survive).