Why does Oracle keep working on MySQL 5.1 2 years ago the first version of Mysql 5.5 was released. What is the reason to continue working on MySQL 5.1 - the current release is Mysql 5.1.62

It looks like there are should be fundamental differences between them. I am confused which version to prefer.
Oracle are obliged to fix security bugs and updates for MySQL 5.1 until the Premium support period has ended - remember that there are paying customers out there using the product in production.

See page 13 of this document for support end dates by MySQL version.

MySQL 5.5 is more than ready for production use &amp; I'd recommend using that for any new projects going forward.