Because they needed good numbers and they got them. They havent had anything to cheer about for a while.
sonys probably not THAT happy about those numbers. Remember that the price cut means they're losing more money on the system than before and when you price cut your system by $100 and still only beat the Wii by 30k units, not good enough. Though im sure they'll spin it that they almost sold twice as many ps3s as microsoft sold xbox's. the name of the game is making money and that spike isn't nearly large enough.
Not to mention the time and R&D that went into a whole new design model that came with that price cut.
They were making a profit on each machine for a while. They are taking a hit with the slim though. I would imagine they would be pretty happy, especially in an economy where there are a lot of people who are struggling. If this was a booming economy I could see how they would be upset. In the current economy I couldn't imagine them being anything but tickled pink. If they are able to sell close to 500,000 through october and november, I would imagine they would be super happy.
Now we're nitpicking. You're assuming it's the exact same system. It's not. If they were just going to take a bigger loss on the same console they could have done it a long time ago.
Still I think that you have to keep in mind is that the Wii isn't necessarily their "direct" competitor. Sure it is a gamming ing system but it has a different type of game as well as different audience. As far as the 360 goes, that is the system I would want to catch first.