It's because AWS runs their control plane out of us-east-1, particularly lambda. There are also some very popular services that can only be run out of us-east-1. It's not really a technological constraint, it's a design decision.

Wait so then how are they signing big customers still who would definitely lose a lot if us-east-1 went down?

(who would be motivated to check if anything is restricted to only us-east-1)