...
Almost exclusively because when edX was started back in 2011, AWS RDS supported MySQL but not PostgreSQL. (PostgreSQL support was added in November 2013). General consensus among the developers at the time was that PostgreSQL would have been a better long-term choice, but RDS offered vastly simpler operational overhead of RDS compared to the other database hosting options available at the time. Using RDS freed up developer bandwidth to work on the new features needed in the startup phase to ensure that edX would survive long enough for anyone to still care about the choice of database.
...