I believe that, at least for AWS, the us-east data center is drastically larger than eu-west.
Larger, older, slower, more fragile. And cheaper.
I've been wondering this myself lately. It seems that every major EC2 outage hits US-East. By comparison, my US-West instances have way better uptime (granted, over a shorter test period). I've never tried the Europe or Asia zones, but I'm tempted to now.