It probably is the most used, being a cheaper alternative to us-west, but are you suggesting it fails more because it is used more? It does seem that the big AWS outages (in the us) have been concentrated in us-east. I have wondered if it just because us-east is newer so they haven't had has much time to work things out, or that the us-west team is a little better?

edit: btw, I am not dismissing "used more" as a valid theory. More use = more hardware = more complexity which could lead to more failures.