theoritically, Fizzer could design 60 player, 70, or even 100 player games just for the fun of it... but the problem is would WL's server be able to keep up with it?
I seriously seriously seriously doubt that server load is the real issue. Even real-time, most of the computation resources required by the game are supplied by the players' computers. Only submitting orders, chat, getting game updates, and perhaps a few other features require the server, and almost all of these are basically just database functions, except for enforcing the rules of the game, which, frankly, is actually pretty simple code (in terms of computational complexity and resource use).
Think of it this way: If there were 50 humans playing in 1 game, compared to if there were 50 humans playing in 25 1v1 games. Warlight already clearly handles the latter. It could easily handle the former.
Now that I think of it, it seems to me one of the most plausible possible reasons for the restriction is simply to put a cap on it to discourage arbitrarily huge games being created for no real reason (i.e. abusing the system just cuz you can). It's like putting a length limit on user names and forum post titles. Yeah, it could probably handle 1000 character user names just fine, but who really needs that for any serious purpose?