I am surprised to have stumped the OP with my suggestion. Let me flesh it out a bit. The question was, "Why do you insist on Americanism when the English language has rules?" Two misconceptions spoil the question by suggesting...
1. ...that English should be used because it has rules. But that's not a valid point, as American English has a complete set of rules as well.
2. ..,.that Americans insist. On the contrary, we don't insist you use our rules--"you", meaning other English speakers, including our own citizens who are of different dialects. When we visit another country, we don't lay down language rules. When foreigners visit here, ditto. In fact, we don't even "insist" it upon our own selves; we use the language we grew up with and make the best of it when face-to-face with differences. We have not had the advantage that is yours--to hear daily your Received Pronunciation. American English evolved over centuries from quite a few non-English languages, as numerous as the English-understanding countries you list. No one is now alive to have tracked its steps. You can surely cite counter-examples to what I've written, but by and large, we speak the language we, the living, were given. Not guilty, your honor.
An aside, helpful or not: I've spent considerable time in England, as well as in Canada, Australia and New Zealand, and I must say, those folks don't sound very much like you guys anymore. At a Brauhaus in Munich in the 80s, someone at a nearby table turned to us: "Excuse me, but we've decided you're Canadians." "Americans," we replied, "but why did you guess Canadians?" "Because you aren't loud." We laughed, grimacing secretly as my wife and I both wished that one implied premise weren't so spot on.
