The laptop and PDA were probably also considered a 'niche' product when they were first introduced, but certainly keeping them under the wrap of the label 'niche' didn't help them to proliferate around the world as they have today.
Let's be clear about expectations. The first portable computer was the Osborne 1. While it did have a 5 inch screen like the OQO, it also weighed about 25 pounds and was the size of a carry-on suitcase. It took years for portable computers to proliferate.
The first PDA was the Psion 1 from 1984. It was over a decade before even the first Palm Pilot came along.
Dating the OQO from the release of the 01 in 2004, we still have a while to wait before it will become mainstream.
Welcome to life on the bleeding edge.