I rather doubt that someone moving "up" from a cheap single or pump to a double is attracted to OU's based on what world champion shooters shoot, any more than I worry about who drives what in NASCAR when I go to the local dealer to buy a vehicle. My guess is, said individual walks into a well-stocked gun store. He sees a whole bunch of OU's, guns he recognizes from the range and/or from magazine ads perhaps, with names he recognizes (Browning, Beretta, Ruger), at around the $1,000 price range, give or take a few hundred. Sxs . . . he may see some very cheap ones, less than the price of a really good pump; and he may see some priced at 2x or more what he would pay for an OU. Plus, with an OU, he'll almost certainly get screw-in chokes. Far less certain with a sxs. Plus, with an OU, he's going to get a single trigger--which he's also used to. Far less certain with a sxs. So, his brain tells him--unless he's been hanging around doublegunshop.com, reading Double Gun Journal or maybe Shooting Sportsman, where he would learn more about sxs--that OU is obviously the gun for me! Plus, if he hangs around the range much (unless it's an unusual club, like one where I hang out and we occasionally have an entire skeet squad armed with sxs), he will have seen lots of OU's, likely very few sxs. He's likely had the chance to handle some of those OU's, perhaps even having shot a friend's Beretta or Browning. So price, familiarity, what he perceives as preferred "options" (ST and choke tubes) . . . why on earth wouldn't he pick an OU?
Popular choice is quite obvious. Facts of performance . . . the vast majority of sxs are field guns, not competition guns. It's darned hard to prove "competitive advantage" in a field situation. I can determine, for example, that the DT gives me a definite advantage over the ST, because I know that every season, I kill birds by instantly selecting the tighter choke--which I could not do with a ST. Whether someone else sees that as an advantage (and there's at least one person here who doesn't) . . . well, that's a matter only each individual can decide.
But these "my choice is better than your choice" debates go nowhere and help no one. 
People should be able to justify their own choices without putting down others. I haven't "declared the superiority of the O/U and the ST," just stated the facts of popular choice and competitive performance - yet that seems enough for some folks to man the barricades. 
