Pure speculation, so feel free to flame- The HP of yore was a test equipment company first, stuffed with old school engineers. They understood and appreciated the superiority of an RPN calculator. They understood the necessity of good build quality and key action. Those people are called Agilent now. The HP of today is a marketing company and my guess is most of them couldn't use an RPN calculator to save their lives. No doubt getting traction on the idea of making something they don't understand and can't appreciate, is very difficult. It would be great if they can bring back the old construction. I'm not a 15C user, being more attached to the later 32S models, but it looks very capable. That 33S model, I assume from Kinpo, was headed in the right direction, but the chevron keys, bad battery life, illegible decimal point and overall strangeness just didn't do it for me. It also had some firmware bugs, though so did at least the first 32S and maybe the 32SII as well.
Though not RPN, I've been carrying a Sharp EL-W516 around for a while. The Sharps tend not to be available locally (TI owns that market) but are incredibly cheap from Amazon. Extremely powerful and if they were RPN they'd take over, but alas I don't see that ever coming. Does HP have some kind of lock on RPN?