It's likely that Canonical developers haven't deemed it stable enought
The Ubuntu Developer Summit that just ended has revealed another piece of interesting information regarding a very important feature, especially for the users of the server edition. Canonical will not be using the relatively new MariaDB.MariaDB is a community fork of MySQL. A few developers felt that MySQL was becoming too closed for their liking and because the license allowed them, they developed a fork called MariaDB. This new fork has already been adopted by quite a few distributions, but it seems that Ubuntu will not be joining them.
“Another big session that we discussed was about the MySQL situation. There are multiple MySQL versions and forks, a cornucopia of different kinds. MySQL 5.5 will remain in 14.04, with Maria and Percona available in universe,” said Jorge Castro, a representative for the Ubuntu Community Team.
This is actually very important news, for the people who are anxiously waiting for the server version Ubuntu 14.04, which is scheduled for a five year support plan. It’s also very unlikely that Maria DB will be adopted in one of the subsequent point releases down the line.