Why did the Americans set up their bases on the east side of Britain during World War 2?

this is a guess but i would bet because they needed to be the shortest distance for flying missions to Europe - most fighters of the the day were short range and the heavy bombers just barely had enough fuel to reach Germany and return - also airfields needed to be close to the Germans intended targets to get fighters into the fight as quickly as possible to defend before the bombers reached their targets