The phone market is doing a new product every couple of weeks, but how about the individual companies? Most buyers are on contract, and can't change phones more frequently than once a year with a reduced subsidy or once every two years with full subsidy.

If a phone company introduces a new phone, and then a few weeks later introduces a better phone, they've to some extent limited the market for the first one to the people who happened to hit the end of their contracts in that few weeks. Given the two year contract cycle, a phone a year makes a lot of sense (assuming your phones are good enough that it takes the competition several months to match or best them--as has been the case for Apple so far).