There are also other historical reasons. It's traditionally the job of the pilots in the plane to keep track of its position, meaning giving them GPS was more valuable than giving position information to the ground. The US is just getting to the point where air traffic control can get GPS from planes, and that is based on ground stations which won't work over the ocean.
Which leads to the final issue, the outdated nature of the rocketry industry (and the intrinsic difficulty of getting to space), has made it very expensive to launch the satellites you need for worldwide communication. They exist, but they are much more expensive than any other way of communicating. Which means, if the airlines can piggyback on ground based comms (which they can), there's not much incentive to make that work over the ocean. Until you lose a plane.