why is PostgreSQL instead of MySQL? What are the advantages of Postgres over MySQL in DBaaS implementation?

I've seen some Database-as-a-service projects and all of them chooses PostgreSQL.

Probably because for the sorts of applications dbaas implementations target, PostgreSQL dominates the market among open source solutions.

MySQL is very popular on the web site, but as a database serving multiple applications from the same db, PostgreSQL has been the de facto choice in the open source world since I have been working with db's (at least the last 12 years).

licensing might be an issue.