American (US) orthography and pronunciation question

[TXlib]

I'm betting Interrobang will have a lengthy answer here...

Doesn't most of the english-speaking world spell "-our" and "-ise" as opposed to "-or" and "-ize" (e.g. -- colour vs color, and utilise vs utilize); also defence vs defense, etc.

When did the US orthography arise, and why?

Also, regarding pronunciation: it seems to me that many Americans tend not to be able to differentiate between australian/english/scottish/etc accents. I assume this is in part, insular in origin (i.e. -- we hear US regional variation much more frequently, and rarely hear australian or british, so have a harder time distinguishing them), but i also think it's in part because our pronunciation is fundamentally different.

Why is this, and how did it come to be?