The Gethenians have a heck of a nature to contend with. If you can imagine Siberia in the middle of February, then you can imagine the Gethen equivalent of beachfront Malibu property. Seriously cold ice age they've got going for them on that planet. This exceptionally dangerous form of nature has had a major impact on the development of the Gethen society for centuries. Their technology has grown considerably slower than ours, and their cities are designed for extremely close quarters to share warmth and other necessities. And on a planet where every step could be a death sentence, you tend to get where you're going slowly. In The Left Hand of Darkness, it's all just one more hurdle for Ai to leap over before he can appreciate what it means to be a Gethenian.
The slow pace of the Gethenian industrial revolution is not simply the result of the fierce Gethen weather. Social conflicts, minor scuffles, and the Gethenians' androgynous nature also played a part.
War breeds technology innovation faster than response to the natural world. If the Gethenian really want airplanes, a war would be just the ticket.