Primarily this was to avoid unnecessary civilian and collateral damage:
"Until Monday, the United States refrained from striking the fleet used to transport oil, believed to include more than 1,000 tanker trucks, because of concerns about causing civilian casualties. As a result, the Islamic State’s distribution system for exporting oil had remained largely intact."
Plus why destroy infrastructure required by civilians during peacetime if you don't have to? That just puts them in a worse position when all this is done.