IIRC, some AWS services are solely deployed on and/or entirely dependent on us-east-1. I don't recall which ones, but I very distinctly remember this coming up once.
CloudFront certificates
Everything new basically, like the AI services.
IAM and Route53 have dependencies on us-east-1.
AWS Organizations/Account management is us-east-1.
And if you want a CDN with a custom hostname and want TLS…you have to use us-east-1.
IAM
AWS IAM has caused multiple cross-region outages.