Metal racks usually only fail on the first tooth where the most mechanical shock happens, that's why all the GOOD pistons are POM or nylon with 3-7 metal teeth (depending on FPS). Metal is highly wear resistant making it the best material for the sector gear to slip off of.
This particular piston was engineered to break, there's no other way to put it.
The rack has no bottom land between teeth, they simply V together creating easy shear points.
Hardened metal is inherently brittle, meaning it's not good at absorbing mechanical shock, but excellent against mechanical wear.
Nylon is soft, and slightly elastic, meaning it absorbs impact and springs back to it's original shape.
POM isn't quite so elastic, but is much harder, and brittle like hardened steel. But POM pistons are engineered with reinforcement (the bar across one or both sides of the rack) allowing it to take higher stresses.