Actually that the Nav'i are technologically stuck mirrors human development. There's an idea that says that the more access a particular people have to natural resources and the more hospitable the environment is (up to a certain point), the less likely they are to develop more advanced industry. If you look at the resource rich areas of the world, mainly the equatorial tropics, you'll see that human culture stalled out roughly around bronze age. Compared to their cousins in more northern climates like Europe or East Asia, there's a distinct lack of progress across the board, textiles, agriculture, pottery, metal working, etc.
The nav'i appear to have no need for technology outside of what they already have and thus having no outside pressure to promote it, they didn't.