I hadn't really collected any 6mm stuff when I was young. I recall seeing a vendor at Fall-in! who was selling Baccus 6mm historicals. I would constantly be looking at all the bases of ancients and napoleonics. Those looked like armies. When Command Horizon came out, it was the motivating force to put together 2 armies and GM games at the local cons.
It was a fun game, but open to too many rules lawyer incidents, (at least the games I ran) which led me to look elsewhere for something fun.
FWC was interesting, but I was making armies with lots of 15mm sci fi I had lying around.
Mini Mech was inspired by: 1. a super long and utterly disgusting game of Battletech, where 6 of us tried to run a lance of mechs each. 2. a fantastic Kaiju city table with foam buildings all over it. 3. throw in a little online mechwarrior city map and viola!
I guess I like the fact that with 6mm, you have plenty of room to maneuver on a small table. It doesn't hurt that they are easy to paint and terrain is much easier to make.
I certainly keeps me happier than 28mm ever did.
In the end, I guess it's play what you like, and I like 6mm sci fi.