SimplyMEPIS used to be based on Ubuntu, but only for one release, MEPIS 6. It
lasted only for Feisty. When Gutsy came out MEPIS 6.5 had reverted to Debian,
Lenny as its core.

The thinking at the time was that Debian was too hard to maintain because they
had no clear release cycle, but they found that many users became confused with
Ubuntu's repositories and they ran into difficulties as some MEPIS functionality
suffered when people got carried away. Also they found Ubuntu's six month cycle
too hard for a little distro to keep up with. There was just enough different
that made it hard to manage. MEPIS does not use sudo for example. It has root
logins. It has many of its own utilities.

It is solidly Debian now and MEPIS 8 looks and works great. I was a MEPIS user
for two years before Feisty came out.