It's simply cheaper, that's what it boils down to, the Ghawar field in Saudi Arabia had production costs under $2 a barrel for light, sweet crude for decades, now that field is down to scrubbing operations and requires barrels of seawater to be pumped in and out again for every barrel of oil. North sea Brent is almost depleted also.
We will continue to use whatever is most cost effective, as fields like Ghawar shut down we'll tap more and more expensive sources until we're down to shale, sand and coal.
And at that point we will be the exporter, but that's a ways off as there's still a lot of oil in Russia and so on. This process is obvious to everyone, which is one of the reasons we're having so much trouble with oil producing nations right now, they want to get what they can for what's often their only resource.