Using multiple rails is cheaper than using a single higher power rail. So, the few power supplies that used a single rail were always high end. People associated the single rail with quality and high end. So you can make a powersupply of equal quality and electrical strength using both a single rail and multiple rail setup, people associate single rail with quality.
That's actually quite incorrect. Multiple +12V rail PSU's aren't cheaper. Single +12V rail PSU's aren't more expensive to build. You actually start with the same platform either way. To make the multiple +12V rail model, you actually ADD additional components to split and limit and in some cases filter the separate +12V rails.
I'm not sure who perpetuated the whole "single +12V rail PSU's are more expensive to make because the one rail has to be 'stronger'" myth, but it's a big lie.
On the retail end of things, single +12V rail PSU's aren't generally more expensive. PC Power & Cooling is generally more expensive and they happen to use a single +12V rail design. They're also heavy into marketing so..... So maybe it's just the fact that the most expensive PSU on the market happens to have a single +12V rail, but that doesn't translate into "single +12V rail PSU's in general are more expensive...."
If you look at most of the OEM's, except Seasonic, you'll see that their base designs are actually multiple +12V rail. Channel Well, Andyson, Enhance, etc. All multiple +12V rail. If they end up as single +12V rail units, that is not by the choice of the OEM and their engineers. It's the choice of the marketing folks and their bean counters. They have the OEM remove the additional OCP circuitry and solder up the output leads into one big output. In the end: It's easier to pitch the idea of single +12V rail than to actually try to educate a customer as to how and why the +12V rail is split and they get to save a couple bucks off the BOM.
Who are you going to trust? Engineers that design this stuff from the ground up or some marketing guy who's going to tell you that his product does magic while some bean counter stands over his back counting the number of items on the BOM.