CrashLemon

I can't tell you why they do not participated but i can tell you that if you look around you will see that a lot of peoples are making some Mod for current game like Tiberium Aftermath a C&C mod for BF2 and C&C Reborn http://forums.xisto.com/no_longer_exists/ a C&C Renegade total conversion in C&C Tiberium Sun.

The way it work is simple. When you enter a game and the map start by co_ is a Cooperative game so you gain level each time you kill enemy, repair etc. When you change map you lose all your Exp or when you change game you lose all your Exp. The max lvl is 10 and in some game they installed mod that enable you to pass the lvl 10. Thats all. Hope i helped you