rkalla's answer is good, but there's also the issue of the US east coast being a good place to deploy applications that need decent performance to the majority

rkalla's answer is good, but there's also the issue of the US east coast being a good place to deploy applications that need decent performance to the majority of the english-speaking world. Dropping your servers there puts them within reach of Europe without as much RTT as the US west coast. Obviously the right answer is to deploy applications into multiple AWS regions, but that's not appropriate for every service's architecture unless built that way from the start or modified specifically to do so.