The portable GPS market was a blip on the timeline; popular for only about 5 years. Several technologies converged to make them possible: public GPS, sensitive GPS chips that didn't require large antennas, cheap solid state storage (this was huge), and high resolution color displays. As soon as they all existed, we could get portable GPS units for $400-800 USD.

Unfortunately for these manufacturers, all these technologies also enabled smart phones with GPS which quickly destroyed the market.