Hacker News Reader

I think WordPress just tries to cut complexity by using MySQL only so it doesn't have to deal with support for arbitrary RDBMS's.

More likely, the code is poorly written and heavily reliant on MySQL.

This is more or less the correct answer for Wordpress-the-ecosystem.

Wordpress-the-base-install has a thin abstraction layer over the database in the $wpdb object.

But plugins and themes can, and regularly do, use PHP's MySQL functions to access and manipulate the DB directly.