One of my favorite PCs in old times were HP Jornada handheld pcs (680, 720) - perfect match of usable hardware keyboard, large-enough screen, protected by keyboard when closed, and usable OS designed for doing tasks and not only multimedia/calls/browsing. All this perfectness was in case, small enough to fit inside pocket (ok, rather big pocket) and was able to work 4-5 hours (psions much longer and from generic AA batteries)
This market was rather large - HP, LG, Psion, Philips and many more, but somewhere near y2k they all just ... gone. Yep there were some efforts from Nokia with E90 but it was "even not as good as 9500" with s60 Symbian designed for different tasks.
I understand that some part of this market moved to qwerty phones, which actually disappeared couple years later. Some part moved to tablets and "business-oriented" phones. Sony made some attempts with it's sub-notebook series but they were too slow with first generations of inlet atom cpus (really, i was surprised that windows 7 took about 5 minutes to start on fresh pc). All modern stores are full of different types of ultrabooks, they all are thin but they all can't fit in a pocket because of large screen.
Nowadays it is so easy to adapt a power-efficient cpu like ARM A53 or something from entry x86 Intels into a HPC-size form-factor with good old proven qwerty keyboard - so why is nobody making one? I mean not entertainment device, but a "smaller than sub-notebook" workhorse