Two reasons:
1) If oil refineries gear up to make more g@$oline, less diesel's available (not sure why, but IIRC it has something to do with the refining process; an oil man can elaborate)
2) Oil companies finally realized that they can pretty much charge whatever they want for diesel...truck drivers are going to pay it regardless of the cost, and "other" diesel sales are trivial in their minds.
Either way, it sucks!! I wish I had the facilities to practically run WVO...unfortunately, as it stands, I don't have the facilities to store or filter it. The good news is, it looks like
algae-bsed fuel is starting to become a reality...