After looking at the first link above and comparing all the features and such, I can’t figure out why everyone uses MySQL or (if they go that route I’d prefer they use) MariaDB. Looking at the chart PostgreSQL seems to be much more feature rich and according to Wikipedia even better meets the SQL Standard.
Having used Oracle so much I can easily see why I like PostgreSQL better. Its much more similar to Oracle than MySQL is.
That still leaves me with: Why did MySQL become the open source favorite? And why does it remain so?