[mkup]

Well, PostgreSQL is licensed under BSD, which makes closed-source, commercial forks possible. That's why it is attractive for large businesses, like Apple.

GPL-licensed MySQL serves the opposite purpose: it is impossible to fork it into commercial DB product, grow it as times goes by, and threaten Oracle. It is nearly impossible to create commercial product for this market, because there's a free product there: MySQL. MySQL is like a ditch around the Oracle stronghold, a ditch to keep competitors far away. GPL is a perfect tool for such tricks.

In contrast to GPL which suppresses commercial developments in it's market segment, BSD encourages them.

For the same reason, Apple slowly gets rid of GCC in favour of clang. It is perfectly OK from legal point of view to include BSD-licensed compiler into commercial closed-source IDE, unlike GPL-licensed compiler.

[sigzero]

I actually think the license had very little to do with its popularity. The fact that MySQL was a lot simpler than Pg made it popular.