Because I'm most at home with Lubuntu.
Lubuntu has proven itself to be just as reliable as Ubuntu.
Every time you go with a smaller distro, there always will be performance or compatibility issues.
That being said, peppermint is a derivative of Lubuntu, which is a derivative of Ubuntu, which is a derivative of Debian.
I did end up with the full desktop, but doing 'sudo init 3' will unload the desktop, in case I need 150 more MB of RAM.
Aside from Peppermint using X11 as the dekstop interface, underneath they're pretty identical.
Edit: I just tried installing the 32bit version, for some tests, and it's not installing on the A-Pi!
Trying out the 64 bit version now...