In short: supply and demand. The price of a 20 ft. container is lower than the container price of a 10 ft. container. There are more 20 ft. containers available on the market. But the matter is a little more complex. Container freight has more than doubled since the stricter rules for emissions in 2020. The ship's fuel may only contain a small proportion of sulfur. It got expensive. Then there was the COVID-19 crisis. Closed factories and ports around the world have caused freight prices to skyrocket. In some regions very few sea containers were available. This market deficit has led to higher prices. In Asia, a lot of new sea containers have been built because it took too long until the used containers were available. It is possible that the prices for 40 ft. high cube new containers will soon drop a bit.