The CPU was some oddball thing much faster than its contemporaries that no doubt cost and arm and a leg. The battery technology wasn't ready so they had to compromise on life. The cellular modem technology was first gen: slow, expensive, unreliable, and without an Internet to connect to. The demo had them sending a fax!
Plus they had to build the OS from scratch with an entirely new paradigm (no keyboard!). And it had the unfortunate first gen pen-interface mistake of focusing heavily on handwriting recognition tech that never worked properly on that generation of hardware. Even today its pretty iffy, and virtual keyboards are the preferred solution. The only company that kind of got away with it was Palm, and only because they invented a new alphabet that would be easier for the computer to recognize.
It also goes to show just how much of design is not only coming up with the ideas, but realizing when the technology has advanced to the point where they can be practically implemented.
I also miss my blackberry keyboard. iOS has never done it for me the same way.
Dick Tracy had a smartwatch, but it took a while to actually bring it to market.
--- Edit: after reading to the end of the article, there were further difficulties: the units cost over $6,000 (adjusted to 2019 dollars), also there were supply chain problems from their competition (not sure how seriously to take this one).