2 reasons. The winter is ending, and with it demand for HHO (home heating oil) is falling rapidly. As HHO is nearly identical to diesel, we are awash in a sea of diesel. I've been diesel since '94 and I remember paying the 'winter tax' every year as HHO demand went up.
Reason 2 is that we are not exporting as much diesel as we usually do. When we changed to ultra low sulfer diesel (ULSD), our diesel became compatible with diesel used in the EU. We've been a net exporter of ULSD for a couple of years now. If you ever wonderd how gas can fluctucate wildly, but diesel stays pretty steady, that's why.
Not sure why we arent exporting as much, or if its only temporary.