It’s mostly the early AWS services that were created before AWS had the concept of regions.

Certain services e.g. IAM (who/what can access what and how) operate out of us-east-1. If your app is hosted in eu-west-1 but needs to assume an IAM profile to authenticate with S3 to read/write data, you’re fucked until IAM is back online.

S3 has regional buckets but the control plane effectively operates out of us-east-1.

You also need to factor in us-east-1 being the default in most tutorials and libraries, and that a lot of these companies are US based so will stick with that default and not bother with multi-region tenancy because it’s difficult to justify the expense.