Because it weighs 6 oz. No PowerBook ever made even comes close in terms of portability. And my rechargeable AAA batteries last for weeks instead of hours. Palms not as powerful as a new Mac, and the screen is much smaller. I think of it as approximately a Macintosh SE in my pocket, because it runs that magnitude of software.
Palms are surprisingly useful given their size and speed.
With AvantGo, I can get selected Low End Mac articles and other web sites to read when I have a few minutes. Browsing in AvantGo is faster than on my PowerBook, because it strips out most of the graphics and formatting. Combined with a mobile phone, I could have wireless Internet that has a much larger range than AirPort has. No wonder Palms have taken the market by storm. If you are considering buying a PowerBook, you might think about these other options like a Palm, digital camera, or older PowerBook combined with a desktop computer.