The quick answer is the learning curve, with 10g Oracle look tame next to Postgres which BTW is the only other RDBMS(relational database management systems) after Oracle with ANSI SQL Time Interval implemented, Microsoft tried in 2005 but the math broke in beta three and was dropped.

There was a free Postgres book online for more than five years it got published and was pulled so you are on your own when it comes to free resources. A while back it was funded by DARPA because it is the replacement of Ingres the first RDBMS.  MySQL hired Peter Gulutzan so most basic features will be available in MySQL soon because Peter had his own database engine he built.