Sorry, I was gone for the day, hope you see this response.

I think all of those play in.  The industrial agriculture companies started giving major funding to the land-grant universities a while back.  So the major agricultural universities don't dare put out research critiquing the conventional system.  I've talked with people whose research showed that NONE of the chemicals they were testing were very beneficial, but they were told that they could not publish that research.  And since the Extension Service is funded by the same people and relying on the published studies, the only solutions they recommend are the conventional chemical ones.  

And then there's everyone with a product to sell.  Holistic sustainable agriculture requires fewer inputs, and the inputs decrease over time -- which means less and less money.  Conventional agriculture requires ever-increasing inputs, in contrast.

And our type of agriculture is difficult mental work!  Farmers have been told they're stupid, and just follow the nice, simple instructions on the bag of fertilizer.  It's difficult to get out of that rut.  

There's also the farming culture.  It's very conservative -- not necessarily politically (although that, too), but in terms of not liking change.  If you're the "weirdo" doing "that organic stuff", you'll get grief at the local feed store, equipment store, etc.

And then there's the government.  USDA and many of our state agriculture departments are terrible examples of industry capture of the agencies.  The senior officials are all from industry, or hoping to go to industry, and so they make the rules to suit the big money interests.