Winter in Tibet doesn't bring any tourists due to several factors. First of all because of the border closure from February till March. The second reason is cold weather and heavy snow. Although the days are sunny and bright and perfect for spectacular views, a lot of roads can be closed due to the heavy snow.
The oxygen content is inferior during this season, especially at higher elevations. It will be harder to get to the famous sights because of all these factors. Although the prices are much lower than during any other time of the year, still only a few people can manage travelling in Tibet during winter.