The real reason is us-east-1 was the first and by far the biggest region, the same reason that new services always launch there but other regions are are not necessarily required (some services have to launch in every region).
The us-east-1 region is consistently pushing the limits of scale for the AWS services, thus is has way more problems than other regions.
The us-east-1 region is consistently pushing the limits of scale for the AWS services, thus is has way more problems than other regions.