The coronavirus epidemic is causing production delays to goods made in China, which has led to Nintendo Switch units selling out on the official My Nintendo Store in Japan.
The novel coronavirus epidemic is currently ongoing in China and is responsible for over nine hundred fatalities as of the time of writing. Parts of China have been quarantined and businesses have been temporarily shut down as a response to the outbreak. As such, a lot of companies that produce goods in China have had to delay the release of items that were meant to be made in facilities that are currently closed down.
Nintendo Switch units are mostly produced in China and the company is expecting delays of Switch units and Ring-Fit Adventure games in Asia due to the coronavirus epidemic. This has caused a spike in demand for Switch units in Japan. According to Japanese Nintendo, the official My Nintendo Store has officially sold out of Nintendo Switch units, including the Minecraft pack-in set. However, there are still Nintendo Switch Lite units in stock.
Nintendo has periodically had issues keeping Nintendo Switch units stocked in Japan, especially during the early years of the console's release. Japanese Nintendo fans struggled to buy a Nintendo Switch when Splatoon 2 was released, due to the demand far outstripping the supply. Nintendo president Shuntaro Furukawa recently had to apologize during a shareholders meeting for their not being enough Ring-Fit Adventure games available in Japan during the 2019 holiday season, as the company had underestimated the demand for the game.
You can't blame Nintendo for the current production issues involving the Switch, as no one could have predicted the coronavirus epidemic or its effect on the games industry. Nintendo doesn't have much choice in the matter, outside of opening up production facilities in another country, and the difference in production costs compared to making stuff in China is so big that the company would likely rather wait than look elsewhere for a place to build Switch units.
Source: Japanese Nintendo