EC2 existed in us-east-1 only for 2 years before it was launched in a second region.

Those 2 years were already enough to pick up some legacy cruft and special cases. us-east-1 is also the first region for various other old services, like s3 and route53.

I think it's also the largest region across many axis, which naturally means it will hit various scaling issues before other regions.

A big reason is it’s the largest region. More servers = more likely to have problems

Not sure if it is related but it is usually one of the first to get or be trialing new features. I believe a few global services are run out of us-east-1 as well.

This is my understanding as well. New features are rolled out in us-east-1 first, then progressively through the remainder of the regions.