So... D&D is not remotely popular in Japan for several reasons.
1) They've got their own TTRPGs.
Popular with the nerdy crowd, they've got a variety of locally produced and published TTRPGs that suit them just fine. Sword World, for example, is one of the most popular TTRPGs there is, and wound up getting translated into Korean. It's currently on it's 5th edition, too! Their rulebooks tend to be more streamlined than even 5e, as well. And contain a number of "Scenarios" instead of long-format campaign books because...
2) They prefer one-shots.
A different lifestyle makes short and fun 'one night' games much more popular than longer format episodic games. This is due in part to the work culture in Japan, with people working various schedules at their jobs not having many 'sync up' nights. Sort of like Americans, but worse. A short scenario over a few beers is much preferred to playing a continued story you only get to be a part of for a few hours every month or two.
3) Living Space.
Japanese homes tend to be much more compact than Western homes. And having a sprawling collection of 8.5x11 books, maps, miniatures, and dice just isn't really reasonable for a lot of people. And having friends over for an evening's game is a fairly cramped affair as well. Which is why most TTRPG groups in Japan don't chill out in someone's house, but head out to gaming cafes and restaurants that cater to a fairly large group of people having a whole "Night Out" for hours on end. And pick up randos to join a session who might not be there next time.
4) Language Barrier.
Vastly less important than the above, since some basic English is taught to pretty much everyone in Japan, but it does exist. Localizations can help, but there's also the American push toward using a fairly colloquial English for explaining the rules even in the rulebooks that may not translate over really well.
5) Replays are -way- more popular.
Replays are books written about people's campaigns and games. Record of Lodoss War is probably the single most famous Replay in the western world. Some get turned into Manga, but a lot are simply presented in a straight written word format and include some of the at-the-table jokes and laughter. Some, like Record of Lodoss War and Chaos Dragon, even wind up being made into Anime movies or series.
6) Two companies basically control the market.
Kadokawa Futures Publishing and Group SNE basically control the TTRPG market in Japan, which is significantly smaller than it is in the West for reasons 2 and 3, mostly. Group SNE sort of paved the way for TTRPGs, but Kadokawa got most of the business in the end. There's not much room in the market for another contender... Though FEAR (Far East Amusement Research) is holding it's own fairly well.
7) One western game is fairly popular.
GURPS. They made a Sword World RPG module for GURPS and it's kinda popular. GURPS is as simple or complex as a group wants to make it, can be played with one or two small books at the table, and is great for one-shots and short-format gaming. Group SNE pretty much produces all the Japanese GURPS content and that's basically the main reason they're still in business next to the market giant Kadokawa.