Developers. A bunch of them were complaining about only having 5GB of RAM available for games to run (again, I really don't know why because that should be plenty in most cases). Besides, price shouldn't have anything to do with it. DDR3 RAM is extremely cheap and adding another 4GB would cost them virtually nothing.
In 2005, just a few months before the release of the 360, Microsoft upped the RAM from 256MB to 512MB, so they've done it before. It doesn't really make sense to me this time around, but eh, time will tell.
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