There's go to be a technical advantage (however small) to the O/U in competition otherwise you'd see the odd SxS competing at the highest levels.
At club level much, I suspect, is down to familiarity and price.
When someone starts shooting, these days they're more likely to be handed an O/U, so that's what they become accustomed to. When they buy their first gun they then buy what feels right to them... and what they're already accustomed to feels right. It can't hurt either that you can buy a new O/U for relatively little compared to a new SxS (cheapest new Beretta 486 is around twice the price of the cheapest Silver Pigeon). By the time you're ready to upgrade, I suspect a SxS feels even more alien.
I used to shoot better with an O/U and hit rates following my first SxS purchase made me wonder whether I'd have been better simply burning the cash. Today, now the SxS fits, I'm shooting as well as I ever did with an O/U but enjoying it more...
