If I had to bet on why Obama's stimulus proposals are being constructed the way they are, I'd bet on the optimistic side with Nate Silver: Obama is staking out centrist territory on stimulus matters because "he wants the Senate Democrats to do his dirty work for him. All of the sudden, the administration, which is about to spend at least $800 billion, gets to play the role of the fiscally prudent tightwads, negotiating against the Senate Democrats." Certainly, that's how you get to a smart and effective stimulus package via a clever media strategy. (In other news, I don't really see how the 'Price Is Right' analogy from Nate's post applies.)
Of course, all of this is really speculative at this point. We'll know what's going on after it actually happens. How exactly the mix of people surrounding Obama turns into policy remains to be seen.