In the case of this gun, it spent many years living in a leather holster under the seat of a truck. Leather is far from the best material in which to store a firearm on a long-term basis, as the chemicals used in its tanning and other processes can often be corrosive.
Leather will also absorb moisture, including protective oils and other coatings placed on a gun to discourage rust.
By the time Dad discovered the rust on this gun, it had gotten quite aggressive and had eaten pits into the steel of the trigger guard. These pits don't hinder the gun's function in any way, but they do mar its appearance and diminish its resale value a bit.
Rest assured that if this gun had been blued instead of stainless steel, the damage would be far worse, and it wouldn't look nearly as good as this after a couple of decades living under a truck seat.
Stainless steel offers many advantages in a gun, not the least of which is that you can polish out minor imperfections without removing bluing, and likewise the fact that minor rub wear will not hurt the gun's finish appreciably, as it would do to a blued finish.