peak oil. we have lots of oil. we don't have much that is easy to extract thats left.
this is why when oil temporarily tumbled down to what, 35 bucks a barrel, all of the productions for oils were operating under a baseline assumption of 70-100 bucks. they shut down. they aren't going to extract it and lose 30 dollars a barrel.
if oil got up to 150 dollars barrel, other sources would become economical.