I found this wonderful article online that seems to have been abandoned for some time now (source below), it's called:

Timing of Entry from PDAs to Smart Phones: The Evolution of an Industry

Despite having innovative and sophisticated product designs, most of the early PDA companies failed. This was mainly because the success of PDAs was dependent on several aspects of enabling technology that had not been fully developed at the time a flurry of companies ventured into this sector. The success of PDAs was dependent on the invention of enabling technologies such as handwriting recognition software, modems, and power and memory miniaturization.


Several enabling technologies had no capacity to support PDA manufacture. Handwriting recognition was ...