There's literally only one difference between the two slip rings (polarity), and if big generators switch polarities, there's the answer. As crazy as it seems, it has to be the why.
It’s a bit crazy that 6000 hours will do it in. I've collected dozens of end-of-life arc lamps, and the anode made of over half a pound of tungsten is beat to hell, and there's never wear on the cathode.
If you compare the flux density of the current through the brushes, I suspect that will play into the equation heavily.
I don't know what the current through the field coils is typically, but when the engine is cold and running at idle, I'm sure there's a healthy chunk of the 1.5 to 2.5 kw output going back into the armature, and those brushes have maybe 1/10th square inch contact patch.
–awr–
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The abridged version of my brush polarity swap.
I measured the difference in the wear and it is about 4x the rate of wear on the one that takes the abuse.