Other than Valve's content, some of which is on Xbox 360 already through traditional means, what does Steamworks bring to XBL that XBL, XBLA and XBLM don't already have?
MS certainly wouldn't want full Steam on the console. What incentive does Microsoft have to allow someone else's digital storefront inside their digital storefront? MS would counteroffer to Valve to just put whatever they want to sell through steam into XBLM, and they can work out a reasonable revenue share. Valve would refuse because they want to control their own store.
As for Steamworks, as far as I can tell, that would just extend a subset of XBL's own functionality to those playing multiplayer games on Steamworks using a PC instead of a console, something MS already tried and failed at with Shadowrun.
That MS failed at it doesn't mean Valve would, but it still would mean putting keyboard/mouse players up against controller players and that spells bad news.
I see the incentive here for Sony. Their online service has two bullet points: 1) Pretty much like Xbox Live, but 2) Free.
Any deal that offloads the burden of developing and maintaining multiplayer features is a win/win for Sony. They get additional features and content without increased costs, which is good because providing the service produces no revenue. It also expands the playerbase for multiplayer games on PSN.
There are just no comparable incentives for MS to allow this on XBL; it's staked out the idea that multiplayer gaming has value, is worth paying for, and it wants to take responsibility for (that is, exercise control over) the way in which that service is provided, and reap the benefits from those who agree it is a worthwhile product.
Come to think of it, XBL is about the least Microsoft-like thing they do.
