For the revolvers in general, it has to do with reliability and usage of specific ammunition to avoid overpenetration. While today we generally consider self-loading pistols to be sufficiently reliable, this wasn't necessarily the case in the 60s and 70s, especially with the aforementioned types of ammo. Additionally, pistols with large-capacity magazines were just starting to become a thing, so a 6-shot revolver didn't actually have that much less ammo compared to most other pistols of the time.

For the scoped version specifically, it had to do with sniping in urban conditions, where you don't necessarily need a very long effective range, but you do want your weapon to not have a long rifle barrel sticking out of a window, giving away your position.

Here's a video by Forgotten Weapons that goes into more detail: https://www.youtube.com/watch?v=P1zEUGck8NE