Ubuntu considers AWS/GCP to be certified platforms so the server is fully tested on AWS as a hardware platform and there are specific kernel/toolchain/user-space optimisations done to make it run well [0].

The advantage of one of the existing distributions over Amazon Linux is that they are available and tested across multiple clouds, hardware platforms and range of software.

[0] https://www.ubuntu.com/cloud/public-cloud this page leads to various documentation on images, optimisations etc.