Not to mention they can usually get the materials and parts cheaper after it has lost it's initial $600 price tag and has been 'out in the open' for a while.
That's the logical side of me speaking. I'd like to say that they just enjoy testing their product's limits and seeing how much they can squeeze into so little, but so does everyone else it seems.
For Sony, once it has become 'old news', their mindset tells them 'it's time to pump out the smaller one'. This has been their behavior since the very first PlayStation and i'm sure this won't be the last of it we see.
LOL! I see your point but sadly companies like Sony enjoy receiving double the revenue from marketing the same product twice.