Why doesn't Japan have nuclear weapons?

They have a no nuclear weapons article in their constitution.

https://en.wikipedia.org/wiki/Japanese_nuclear_weapons_progr....

Because the US are in charge and prefer to be the ones having the nukes and to tell others "don't you worry we have your back"...

Because the US has nukes, and promises to defend Japan against hypothetical future Chinese aggression.

I think it was during the GWB administration that the US suggested it might move its nukes out of Asia. Japan (and S Korea and Taiwan) hinted that in that case they might seek to develop nukes of their own. The US then backtracked.

The United States wrote the Japanese constitution after unconditional surrender after WWII. US had incentive to ensure a pacifist Japan, after losing many thousands of troops against the IJA. It has now permitted Japanese rearmament, and likely would support Japanese nuclearisation.

Besides other reasons, if you have them, you will be the first to be attacked. If a nuclear war breaks out, why bomb Japan?