Just a guess - as a sometime lawyer - that there must be a patchwork of regulations around the world concerning contests (what is permissible, what is considered to be or not to be gambling, etc.) and peripheral issues (such as whether prizes are taxable and if so whether tax has to be withheld, etc.) It might also just not be feasible to research all of these requirements all around the world. Even if you could, it might be impossible to set up a single set of contest rules that would be permissible in every country at the same time.
Where travel is involved, presumably the sponsors want to limit the cost (e.g. flights to the US Open from within the US, not from Antarctica).
Sponsors might also be attempting to target specific markets - thus, for example, we see free Dove golf balls being given out only in the US because presumably this is a marketing campaign targeted to the US.
That said, WGT should still try to have prizes available to as many competitors as possible, and hold their sponsors' feet to the fire, explaining that this is the World Golf Tour and is designed to cater to players from all over the world.
Again, just a guess. The question has been raised before on the forums, but I don't recall ever seeing a clear explanation from WGT.