According to The Wall Street Journal:
What has fueled the lobbying surge is that President Barack Obama has left the details of the health overhaul to Congress. That means interest groups have plenty of decision-makers to badger. It also makes any legislation vulnerable to death by a thousand cuts.
Noam Scheiber gets in some good hits on this theory. But most of all, let’s recall that Obama didn’t decide to leave the details of the health overhaul to Congress. That’s just how American political institutions work. I heartily agree that this isn’t the best way for political institutions to work; there’s a lot to be said for a system in which the executive (which is less hostage to parochial interests and possesses more policy expertise) to write proposals that the legislature can either accept or reject. But our institutions don’t work that way, have never worked that way, and couldn’t be made to work that way without scrapping the whole constitution.
The President has a very powerful ability to set the agenda, and when his party controls congress to even outline the broad contours of public policy. But congress writes the laws. That’s where laws come from.