Because ISIS would take the oil by force and sell it, generating cash to rebuild their forces.
Currently, there is nobody else who is trustworthy or strong enough to defend these oil fields.
That’s called rule by a crisis and lies.
Lift sanctions from Syria and put sanctions on Turkey for violating Syria’s integrity and Syria is strong enough.
Even without it Syria with Russian help is more than capable to take and hold the fields. They even capable to wipe out the limited US contingent be get there but don’t think it worth the fallout.
ISIS is nothing now.