It's United States Agriculture. Therefore, by definition, it has everything to do with subsidies.
An acre of midwestern land will produce , what, five times as much corn as canola?
Possibly that much.
However, the corn from that acre will yield in Ethanol only a tiny fraction of the BTUs that the same acre would yield in BioDiesel made from Canola. We are discussing BTU's produced, not harvest weight.
Ethanol comes from fermenting sugar. BioDiesel comes from transesterifying vegetable oils. There is a lot more energy in oil and the resulting BioDiesel than in sugar and the resulting Ethanol. And a kernel of rapeseed contains a hell of a lot more oil than a kernel of corn contains sugar.
May I guess that you've just gotten carried away with your poetic license? (or you've never really seen a rapeseed)
An average acre of #2 yellow corn will yield about 416 gallons of ethanol, while (from what I can find on line) an average acre of canola seed will yield about 127 gallons of oil....plus other by products such as gluten or DDG's. High extractable starch corn will do even better, but is generally only grown on contract.