Why are so many MPP solutions based on PostgreSQL instead of MySQL?
It is mostly a license issue.  These developments end up patching the code quite heavily, so if you were to deal with MySQL, you'd either have to open-source your code or be at the mercy of MySQL's corporate owner for the life of your business.  Some offers for MySQL get around that by implementing their work as a storage engine, but that doesn't offer all the flexibility that they need, and they invariably end up patching the MySQL core as well.