to answer maxi29's question...the answer is simple. There was already a population in place that spoke a langauge other than english. Pronunciation of those words would have slowly changed how people speak.
The Amercian Indian laguages do not use the same vowel structure, hence, Brits going to the States would have been influenced by this usage. Over a couple hundred years, a new "accent" would have emereged.
But being American, I love to make Brits mad by stating that the Ameican usage of English is, in reality, un-accented (excluding regional accents like NY, Texas, Southern, Western). I mean you can't even spell color right!!!