Why are so many MPP solutions based on PostgreSQL instead of MySQL? Astor Data, Greenplum and GridSQL all allow Massive Parallel Processing of SQL queries. They are also all built around PostgreSQL technology. Is this just because of licensing issues or are there other reasons? To me, it seems like the MyISAM, not being ACID complient and therefore not running into the same issues with MVCC (like seen here) as PostgreSQL is far better suited for building high-performance data warehouses. After all OLAP load does not require transactions as far as I can see.
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.