I honestly think it has a similar type of relationship as to why Intel / AMD release processors at X GHz when they have capability of X + 20% GHz readily available.
They don't want to miss out on the incremental revenue of the 3.2MP line ... then in another 18 months we'll see 5MP / etc.
That, plus the fact that when designed I'm sure 3.2MP was probably the best price / performance they could get to fit the desired pricing model.
