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).
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).