As far as I understand, the US produces more Diesel than it consumes. A blanket export ban would mean that refineries would have to shut down because they can't get rid of all the Diesel they produce. But since refineries produce more than one product, this would affect other oil products as well. That doesn't sound like a situation you want.

But this is based on my simplistic understanding of how this works, I'm really not sure about the details.