I shot mostly sheet league for a number of years. I'll give you my take on this. I was a pump man for a very long time, but turned to the over and under for a number of reasons. When shooting or playing clay pigion games you shoot a lot, a whole lot more that 99% of the field shooters out there so little things mean a lot.
1: An O/U has fewer moving parts, fewer parts to wear out, fewer parts to break. Generally speaking a quality O/U will outlast a mechanical gun like a pump or semi-auto by a large margin.
2: A pump or Semi-auto has all of its moving parts open to the world. A world full of dust, dirt and unburned & burned powder. All of which needs to be cleaned out eventually.
3: O/U's don't jamb ..... well lets say that they very rarely jamb. Reliability is a big factor.
4: This will sound petty to some but in some eyes its a very serious matter. O/U's don't fling empties all over the place. Many clubs have a rule that if a shell hits the ground it belongs to the club. This is rarely strictly followed but like golf skeet can be a quick game and waiting for someone to pick up empties can be trouble some.
Now that was skeet, trap is another matter. You can get your head handed to you by old timer trap shooters if your shells are constantly flying here and there. For a lot of trap shooters trap is a rythem game. Anything that throws off that rythem is a multiplied irritant. A trap shooter wants to get a rythem going. Each shooter loads in rythem, each shooter closes their gun in rythem, each shooter calls for the bird in rythem. Bouncing an empty into another shooters box destroys that rythem, talking destroys the rythem, talking, gun jambs, broken birds comming out of the house, a poor thrower, man darned near anything can be a problem.
5: Safety, when an O/U is cracked open, everybody can see it and they know that it is relatively safe.
6: And lastly for me, nothing points, swings or carries like an O/U, absolutely nothing.
