Cost is/was reduced from using an aluminum alloy lower. Swiss Arms (SAN) uses an aluminum alloy lower also (although not the same one).
The problem with SIG Sauer USA is that their plan was always to produce cheap imitations of the Swiss rifles at the lowest possible cost, then sell them for slightly less than the Swiss rifles. This gave them massive profit margin, but they weren't too concerned with quality.
Re-designing things, and coming up with a whole new piston AR in the MCX, has allowed them to continue using low-quality (inexpensive for them) materials while suffering fewer parts failures (at least until the current MCX debacle anyway).
The original SIG SAN designs, just like original dimension AR-15 bolts, are not tolerant of anything less than high-quality materials. When you get to make things larger (or with less stress load), then they don't have to be as strong. That's an engineering fundamental.