People are talking a lot about batteries in this HN discussion, but there are industries that are so much simpler to run on burning gas/coal (steel, petro-chemical, etc.). Japan has massive heavy industry, similar to Germany, that 100% depends upon imported methane. Someone also told me (not sure if true) that the infra required for hydrogen is much more expensive than methane because it is "more leaky". Thus, equipment tolerances are way higher for pure H2 vs CH4.
Last: The "we don't like nuclear" view will only last until the next energy crisis. The current PM (Kishida) is already talking about re-activation. Yes, there will be billions to spend to upgrade existing reactors, but Japan never shies away from infra spending! It doesn't get much attention in the media, but plenty of infra (bridges, tunnels, etc.) was upgraded after the 2014 earthquake.
And, FWIW, Japan plans to use high-temperature nuclear reactors to produce hydrogen with very high efficiency.
See: https://www.jaea.go.jp/04/o-arai/en/research/research_03.htm...
Nuclear is unpopular since Fukushima, and Japan doesn't have great renewable resources. The country is extremely land constrained -- and although the "renewables require too much land" argument is silly on the American Great Plains, it does hinder Japan.
Additionally, the solar irradiance is mediocre, and their offshore wind would have to be truly floating wind turbines (which are more expensive) because the continental shelf drops away so quickly.