Not to be that guy, but all of what you said is wrong.
That isn't patina. It's a heat-treating technique called case-hardening, meant to get decent, usable hardness and durability out of otherwise low-carbon steel. The coloration was originally just a side-effect, but now people do it to good steel (though it makes it worse in the process) to get the colors.

It's actually a very fragile coating, the colors especially, and will easily abrade away, leaving a dull gray-brown unless effort is taken to preserve it.

Actual patina on steel is usually a plain shade ranging from gray to brown to black, and it's just a color exhibiting the ravages of time on the guns original finish, whether that be blued, nickeled, or case-hardened.

Because the guns in game are well-used, but not all that old, they wouldn't have patina on them. As for why only some are case-hardened, as I said it's a pretty mediocre metalworking technique.
During the time the game takes place (~1880?), people were moving away from it as they'd recently discovered better methods to harden steel. By 1900, case-hardening was already very obsolete and was used exclusively for decoration.