One reason is cost. The other i think is the number of services.

If I recall correctly, some dns services are in us east only only u host it in other region.

https://docs.aws.amazon.com/general/latest/gr/r53.html

Name registration is in us east only. DNS resolving is in a few specific regions

Not OP, but that was the reason for various previous employers.

Current employer it's all in ap-southeast-2 because of data sovereignty concerns.

Getting the Melbourne AWS Region up next year[1] will be good for redundancy.

[1] https://aws.amazon.com/blogs/aws/in-the-works-aws-region-in-...