The reason for this is that the optimum strategy for victory is the least interesting one - lurk out of the way until only one person is standing, and then jump them.
I think that a mass combat format ought to reward agression.
As such, I like the following:
Circle of Carnage.
Everyone has (e.g. three) global hits, and does single.
When your hits are gone, you are dead, and take no further part.
The fight ends when one person is still standing.
So far so familiar. However...
One point is gained for each hit you take off someone else. A further point if gained by the last person standing for each hit they have left. The winner is the person with the most points, irrespective of if/when they died.
E.g
A,B,C and D each start with 4 hits. A starts by taking a swipe at D, landing a blow as D scuttles away. A and B gang then up on C - A inflicts 2 blows and takes 1, B inflicts 1. C is dead. D watches and lurks. A then attacks B; 2 blows land each way, killing A. B then attacks D, landing a blow before losing their remaining hit. D is still standing, on one hit. A has 5 points, B 4, C 1, and D - despite being the last standing - only 2. A wins, despite having gone down earlyish, due to being aggressive.
Suggested refinement based on what I thought you were saying in the first place and wanting to balance rewarding attack and defense:
Every time you (hit someone | kill someone) you get an extra hit point. The winner is still the last person standing, but the person who ran around killing everyone probably has an advantage over the person who stood around lurking because they have more hit points.