China is actually by far the biggest producer of potatoes in the world, but you're right that on a per capita basis they're not among the top consumers. Per capita consumption is around 57 kg/person/year, compared to say the UK or Germany, which average around 100 kg, or East European countries (Russia, Ukraine, Poland) where it averages around 130 kg.

One reason is that in most of Asia, including China, potatoes are not a staple crop, they are treated as a cash crop. This has to do with a couple reasons. One is the potato growing season, which in temperate climates is the summer, so it's harvested just before winter sets in. In tropical and semi tropical regions, potato is a winter crop that is harvested in spring.

This makes a difference in that potato is a moist crop unlike grains and cereals, and therefore spoils more easily in storage. Europe has a huge advantage in having an extensive refrigeration system for storing and distributing produce year round. Poorer countries either don't have this, or are only now in the process of developing such systems and their availability is spotty.

Even prior to refrigeration, there's an advantage to harvesting just before winter, because farmers figured out that you can store potatoes underground in caves or cellars, and they'll last through the winter. But you can't store a spring crop through a tropical summer. For this reason, in many warmer Asian countries, potato remained a seasonal crop which becomes rarer off season because of poor storage facilities.

If you look at the potato growing regions of China, you'll notice that it's almost all inland, usually on poorer soils. The rich coastland is usually reserved for rice and wheat, their two staples. You can see an even more extreme disproportion if you look at potato growing regions of India, where almost the entire growing region is the belt of the Indo-Gangetic plain in the north.

Keep in mind that India is the second largest potato producer in the world after China, so what that distribution really tells you is that you really can't generalize about how much potatoes Indians (or Chinese) eat. In the north, they eat just as much potato as most westerners, probably, but in the south they don't. In China, the inland regions, specially near Mongolia produce and consume a lot of potatoes, but around the coasts not so much.

It's also important to distinguish between regular potatoes and processed potato foods. As western fast food chains spread across Asia, potatoes in the form of fries or mashed potatoes or hash browns are becoming increasingly popular, and this has massively increased the consumption of potatoes in recent years. In fact, in China, potato consumption almost doubled in the early 2000's because of the proliferation of fast food chains.

TLDR:

• The growing season makes a big difference to potato popularity. Potatoes harvested at the end of summer in temperate climates store really well over winter, which encourages potato production. In tropical climates the season is reversed, which makes potatoes a more seasonal crop rather than a staple.

• Lack of a refrigerated chain from storage to distribution affects Asian countries more than European countries.

• Despite these differences, China or India aren't far behind in potato consumption, and they are rapidly closing the gap. This is driven both by better refrigeration and by the increase in processed potato products sold by fast food chains.

A related question: why are sweet potatoes so popular in China compared to regular potatoes?