I was on a boat recently where the majority of divers had BP/W rigs, and one woman who had a traditional jacket-style BCD asked one of the divers how comfortable their BP/W could be, given that one's back is typically up against bare metal, and the shoulder straps typically have no padding either. She clearly didn't see the appeal. I think comfort is a big consideration for many resort-type divers. They want to dive in a (sorry to use a tired metaphor) Cadillac with cushy padding everywhere, not a Formula 1 car with bare metal sticking out, even though the latter may give a more "optimal" (performance-wise) ride. Keep in mind that a fair percentage of divers are old geezers.
I think typically people are diving in a wetsuit, and that constitutes the padding. If not diving in a wetsuit, there are padding "kits" that will pad the straps and the backplate (I know Halcyon has a system, I'm sure other do too).
However, I have worn mine without a wetsuit (or dry suit) and don't find it uncomfortable. My BP/W actually came with the padding, and I removed it.