InstaGib would break the balance of the game. Even considering that both humans and aliens only need one shot, the humans would still have the advantage of having long range weapons. Aliens would simply be killed on approach. Also, InstaGib sounds more like CTF.
The main point here is that it falls out of the philosophy of the game. So lets leave InstaGib for Unreal (and CTF as well, although gibs flying over the place would be fun).
Of course you can still make the mod, but will anybody use it? Of course, proposing it for trem servers would be much worse. As far as I'm concerned, plain trem is enough. Trem is meant to be a strategy game, not a brute force one like unreal.