the main reason is rust and longevity. Something you might be intimately acquainted with, looking at your sig.
Depending on how you use your car, that one mild steel might turn into rust quite quick, even if its painted, because you can't paint from the inside out...
They also sound different, as i understand it, mild steel has a dull, softer note, while stainless has a louder, harsher note (obviously this is highly depended on the muffler/resonator set up)
Its fine if you want to go for an cheaper product, just don't judge us for wanting the opposite.