On why security companies use them so much, I believe it is a combination of perceived need, public perception, training standards, legislative mandate (in some locals), insurance concerns, and budget. YMMV depending on location.
I'm familiar with the training regimens, and they're not very comprehensive. If the agent isn't already a shooter when they take the job, most companies' training isn't going to make them one. NRA Basic Pistol is more thorough than the training most of these companies give. Basically, the idea is to keep the rounds on a B27 at ten feet without hurting anyone.
As such, it doesn't make much sense to issue expensive weapons to most applicants. I have one agency that comes in to my shop once a year to drop of their service weapons for our gunsmith. The idea is to drop off 15-20 guns and out of that, restore at least 5 to working condition.
Most of the guns turned in are horribly neglected. Alot of Model 10's and 65's. In many cases, the corrosion is so bad that the internals are useless, and most have likely never left the holster during their whole time in service. Judging from the treatment of the weapons, I wouldn't be giving out Les Baers, either.
I've been called to the aftermath of a shooting involving Brinks. Of 14 shots fired by Brinks guards, two hit their target. This was at 4 and 15 feet, respectively. What's more, they fired from the parking lot and sidewalk, with a crowded store serving as backstop. It's a wonder no customers or employees were hit.
Weapons in that case were a Ruger P-series and a Ruger Security Six. Both hits were made with the Ruger. I once asked a Brinks messenger what he was carrying, and he took it out of the holster, looked it over, and informed me that he wasn't sure. (It was a S&W third-gen auto)
The cops I've seen carrying revolvers are generally a different breed. They've had to jump through some hoops to retain their preferred guns, and those guys are really good shots.
Some folks carry revolvers because that's what they're given. Others do so because they've had to earn it.