Former CIA director Michael Morell said last week that the White House has hampered the intelligence community's ability to attack oil fields and oil trucks controlled by ISIS. Morell said that, “we didn’t go after oil wells, actually hitting oil wells that ISIS controls, because we didn’t want to do environmental damage, and we didn’t want to destroy that infrastructure." He went on to say that there was a pretentious feeling in the war room that bombing ISIS oil targets would indeed cause environmental damage.
On November 18th, Colonel Steve Warren said that striking oil trucks owned by ISIS may cause harm to the drivers who may be innocent.
So, this is a decision that we had to make. We have not struck these trucks before. We assessed that these trucks, while although they are being used for operations that support ISIL, the truck drivers, themselves, probably not members of ISIL; they're probably just civilians. So we had to figure out a way around that.
An entire script of Colonel Steve Warren's press briefing can be viewed here.