That would then give either a very lax regulatory body or a very strict one. Then we also have the matter of sovereign states being dictated to by an external power. Most countries resist this kind of 'external interference'. Also, having different regulations means the licensing costs differ, providing choice for both provider and player.
Now consider that the US does not like Internet gambling, it seems. And I'm sure he French would not like to have the Americans tell them how to run their operations. That's not even starting down the path of horse trading politicians dragging the entire process to a standstill.
How about a single model of motor car? Perhaps a Trabbant!
Much better everyone has their offering and we can choose which suits us.
Regs.
Iain.