Why are expressions like “gonna”, “wanna” and “shoulda” American English?

Solution 1:

I'll paste the relevant part of Postal and Pullum's (1978) article Traces and the Description of English Complementizer Contraction (Linguistic Inquiry 9:1):

Why American? That is very difficult to say! Rarely does someone have a good answer for why a specific sound change happened at a specific time for a specific speech community. We can only say what sorts of thing tend to happen, and what sort of things are rare. In this case, you see that the verb+clitic combination forms a trochee, and the unstressed syllable is the one that gets reduced. It would be interesting if a stressed syllable were the one to get reduced. In fact, we'd even start to question whether it was truly stressed.