I think because brass is smoother and wouldn't gouge plastic as easily.
They are a bit thicker than the genuine ones (maybe 3 or 4 genuine to the bunnings ones I use) but they actually get the top end down.
That Mustang Vortex I bought a couple months back - came with a genuine washer in in - idled at the speed I would normally mow with and revved out closer to 3700 (no air leaks). Put in one of my washers and top end come down to 3100 something, but I will have to do the mod for the idle stop screw as it idles so low it will die.
But I am the only one who uses it so it doesn't matter for the moment.
