I find it interesting that people are saying sizing isn't an issue with jackets. It is a BIG issue. If you have the wrong size jacket, you simply can't adjust it to fit, and the tank will be unstable on your back. On the other hand, a backplate with simple harness is almost infinitely adjustable. We have our student rigs set up so that they can be adjusted very quickly, using plastic buckles instead of triglides to set the crotch strap, for example.
The things I see newer divers wanting, that makes them look at standard BCs, are integrated weights, pockets, and padding. There is a pervasive and profound loathing of weight belts, and although there are weight harnesses that accomplish the same thing as integrated weights, they are an additional thing to purchase and to don (and to remember to bring to the dive site!). Pockets can be on one's exposure protection (and generally work better there), but again, wetsuits don't come with them, so you either have to buy pocket shorts or buy pockets and glue them on, or have them sewn on for you. Another expense and hassle, when you could buy a BC that comes with them. And finally, the heavy tank and weights hanging from the shoulders are uncomfortable, and there is a strong perception that padding the straps and back will make that better (which it really doesn't).
One must also acknowledge the convenience of quick releases for getting out of gear. Although a properly adjusted harness is not difficult to don or take off, it's not as easy as popping a clip and just stepping out of the rig.
It's easy for me to understand why people gravitate to jackets or one-piece back inflate BCs. The advantages of a backplate system are only obvious to someone who dives a LOT and begins to value having very few things that can break, and a modular system where any piece can be replaced on its own (or repaired). In addition, if no one has ever talked to you about weight distribution for proper trim, the value of having five pounds of your ballast on your back is not obvious. And if you dive in the tropics and carry very little weight to begin with, the advantage of a BC with no intrinsic buoyancy is not compelling.