The EPA said that, basically, since we're all using less gasoline, maybe we don't need as much biofuel (PDF):
Reactions to the delay are decidedly mixed. A number of ethanol producers rejoiced, including some in Brazil, since any action that could reduce the ethanol content is being put off. The Renewable Fuels Association says the news is a "cloud of uncertainty with a silver lining" and blames the oil industry for not promoting higher ethanol blends (E15 and E85, for example) in the national fuel supply. On the other hand, the National Biodiesel Board says that the EPA is hurting the industry with the delay, since there is no statue-based biodiesel standard and the new levels should have been published in November 2012. Iowa officials generally agree.
The proposal has generated significant comment and controversy, particularly about how volumes should be set in light of lower gasoline consumption than had been forecast at the time that the Energy Independence and Security Act was enacted, and whether and on what basis the statutory volumes should be waived. ...
EPA has been evaluating these issues in light of the purposes of the statute and the Administration's commitment to the goals of the statute to increase the use of renewable fuels, particularly cellulosic biofuels, which will reduce the greenhouse gases emitted from the consumption of transportation fuels and diversify the nation's fuel supply.