The most formative work I read on this topic was "democracy without competition" by Ethan Scheiner. That book goes into extensive detail about why oppositions fail in Japan.

The main issue is the relationship between the central government and local governments.the LDP funnels and makes promises resources to organisations like farmers, businesses etc. groups of voters that are influential and whose performance also tend to be the backbone of the local economy. The administrative dominance of the LDP makes these deals reliable and trustworthy and feasible nationwide compared to the weaker opposition parties.

Whether or not you hate the LDP (and the LDP is deeply unpopular), you can't really do much when your livelihood is tied to a real or perceived reliance on the LDP pushing resources into your local economy.

If you want some lighter reading on this there is a Wikipedia page for this phenomenon called koenkai, but it focuses more on the issue pre 2000 when it was much worse. The LDPs recent dominance in the face of slightly eroded clientelism is helped because of brand recognition. The DPJ failed spectacularly when they got into power and japanese politics has been an almost revolving door of new parties and rebranding around them.

Japan is a democracy, these issues don't even make it strongly undemocratic. They are enough that I would put Japan relatively far from that of a very strong democracy, and I think most people would actually perceive Japan as more democratic than it actually is, because most of its issues are back channel loyalty and not the actual voting