60/40 brass has a density of about 8250 kg/m3 as opposed to mild steel at about 7850. Maybe the extra weight would help.
However I believe part of the issue is related to the fact it is a brass seat pressed into plastic; you can often see they are crooked. This is where viton shines as it will help seal a crooked seat.
There is also the issue with making the brass ones; stuffing around with a lathe with a piece of brass round stock. Even though I love 2 strokes, its just easier to throw a new carby on a chonda and put that on the deck
