The basic reason for this is that it is much easier for a society to borrow a technology (and crops are a form of technology) than it is for that society to invent it on its own.
It may be that wild forms of wheat were available in many parts of the world other than in Mesopotamia. However, once the Mesopotamians had domesticated wheat, it made much more sense for other societies to simply borrow it from them. Instead of having to find the appropriate seeds and spend a long time selecting the best ones so as to get a truly domesticated wheat variety, it would have made much more sense to simply buy seed wheat from the Mesopotamians.
Because borrowing is easier than independent domestication, the diffusion of wheat from Mesopotamia made it unlikely that other areas would domesticate it on their own.