Money is the obvious answer. Also, pretty early on Sony said their focus was their exclusive content vs third party. I'm not saying they're getting that right, but I got the impression that they were looking to more of a Nintendo model where they look after their own IP and let third parties decide what to support.
I think mostly it's just an acceptance (no doubt a reluctant one) that sales of PS3 coupled with MS funds doesn't put them in a good position to fight. If 360 had a much smaller install base they'd no doubt be pushing the developers hard to favour PS3, but that isn't an option currently and to be honest they'd just be wasting money in the current environment trying to outbid MS.
Try to be reasonable... its easier than you think...
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