Question

what geographic factors explain the similarities and differences between American and British English?

Answer 1

About 4 thousand miles. 8 hours on a good plane.

Comments

Try not to use a 737 MAX.

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You forgot time

Answer 2

The US is geographically a long way from england.  There are no language police keeping it pure.  So we have natural evalution.

Answer 3

There are really no Geographic feature per se. The is time and every thing impact ing the evolving local evolving cultures. 

Linguist who touch on remote locales which have similar language patterns to Old English,  Think Outerbanks Islands, Skye Island etc.  Now the the English colonies in North America evolved differently than the the UK.  Our language patterns differed consequently.

Now the lack off communications technology from the 16000's to lets say 1900 aided this process.  Here is an example

To Table a subject

US = To defer to later

UK = let's deal with it now

And the list goes on