I also have wondered why I see so few stand ons, but down south that might have more to do with the varying grass types vs the fixed decks formerly the norm on them, and some other technical factors.
But I think that if you do omit factors like "fun to use" or "less tiring" and only stick to pure production numbers, you're missing an important element. Be it an owner/operator or employees, a machine that is more pleasant to use may actually get more work done in the big picture. With employees if you keep them happier and less fatigued, they will be more likely to stick around. An owner can always use more energy and less wear/tear on his body and any machine that provides that is a lifesaver. I know my limiting factor isn't hours in the day, it's hours of effort in my body. At some point you just want to head home. If a machine can postpone that an hour a day then it's more than worth it.
Walk behinds are, when you are young, only subtley harder to use I think. When you get older and everything aches you notice it a lot more. My impression is that standers are quicker in most situations, but mostly they seem to require less body effort than a sulky/wb and of course less effort than walking. A ZTR may be better on wide areas, and is still easier on the body than a stander (again this shows more as you get older), but the more turning you do the more a stander may make sense.
Large companies are probably more set in their ways. They also may feel the stander takes more training or rather re-training. If Juan has used a pistol grip fixed deck wb for 5 years, he's someone the busy owner or manager can forget about. But buy a new fleet of standers and there is a learning curve.
Also a wb is the most versatile mower, ultimately. It can mow anywhere. A stander can mow MOST anywhere, but in some conditions you'll still need a wb around. When equipping a fleet of trucks with machines, they may feel it's cheaper to have 2 types of mower rather than 3.
Finally we know that in economics the cheaper the labor supply the less capital (equipment costs) is spent on production. If a typical laborer cost $25/hour I'm sure they would spend more money on getting that 10% extra efficiency. But whenever labor is cheap, you often find cheap tools along with it. I quite often see large crews mowing properties where 2 guys will be mowing a pretty darn big area with puny 21" mowers, and a 3rd will be waving a string trimmer around trim-mowing patches of lawn! The other guy is on a ZTR. So they are what is called "undercapitalized". Cheap labor makes this strategy easier than if your labor costs (yourself, if solo) are much higher. Meanwhile, I'm a solo operator and have 2 ztrs, 2 midsize wb's, and 2 decent 21" mowers so I always have exactly the best machine for every property. Because tiny improvements in efficiency are worth more to me than someone with a $10/hour guy using it.
A small company or solo operator is more concerned with productivity (and comfort, and ease of use, and fatigue) than a big company with employees for that reason. And standons aren't cheap, so I would say you'd be more likely to see them used by smaller companies and solos than big fleets.