Laws and regulations

If you try to duel with AK-47's or M16's you are prone to more collateral damage. And don't get me started on UZI's.

And that's just late 20th century tech. 21st century weaponry is bound to increase someone's capacity to take down a lot of people more efficiently.

Revolvers, on the other hand, are 19th century tech. The first patent for a revolver is from 1818. They don't have too great a fire rate when compared to a fully automatic 21st century weapon and it's extremely hard to modify them to fire faster. They also take long to reload even if you cheat with those reloading accessories. So the odds of someone going spree after a duel are less likely.