time is critical in dangerous game hunting, as is reliability. SxS guns are faster to open and reload, and are inherently more reliable.
imagine you are a euro hunter in the deepest darkest africa, and your gun breaks down on safari. what do you do? big problem if you are using an OU with integral locks, so good thing you have a spare gun. no problem if you are using a SxS with removable locksets, because your gunmaker provided spares that you can swap in with a small screwdriver in 3 minutes (he provided the screwdriver, too).
people looking at new boxlock SxS will not understand this concept, as a boxlock is a single component housing both firing mechanisms. sidelocks have seperate firing mechanisms for each barrel that are easily repaired or replaced. this is why sidelocks remained the choice in the veldt even after boxlocks appeared on the market in the later 19th century. sidelock guns are a direct design descendent of the flintlock mechanism, with the firing pin a seperate component contained in the breech.