The reasons are pretty simple
Because America has a lot of long flat dead straight roads, they do more highway driving etc and american cars are traditionally larger sized and engined, automatics were what made more sense.
In europe because the roads are traditionally windier and narrower and cars smaller and lower displacement manual made more sense.
And thats pretty much why, its changing a little these days, but those are the historical reasons that have stuck.
I have both, I like the auto in my commuter, and manual in my "fun" car, they both have advantages and disadvantages.