Nice question! Albeit one that has not gotten answers yet in its previous incarnations. I’ve been thinking about an answer for some time and it is either too long or meandering as it is an extremely complicated question, but here is something condensed and with many points glossed over. The overall summary is that the LDP had a strong start and with the electoral system, political structures, bureaucratic power, and institutional controls the LDP maintained a base in the rural areas.

So first just a few base facts about the Japanese political system. Japan has a unitary government system like the United Kingdom (UK). This governance structure gives a lot power to the central government than for instance the federal system in the United States of America (US) where states’ rights has a lot of weight. This is important later especially in regards to finance. Example is taxes in Japan are very consistent from prefecture to prefecture such as sales and income tax. Also Japan has a parliamentary system like the UK where the Lower House (衆議院 shūgi’in) forms the government. The parties elect a president who shall take prime minister role should the party receive a majority of seats in an election. So it is not as direct like the US system. In addition should any party fail to reach a majority, a coalition government could be formed with other parties sharing governance.

So first the nutshell history of the Liberal Democratic Party (LDP). It was a merger between the Liberal Party (自由党) and the Democratic Party (民主党) in 1955. So in the post-war there was no full majority party. There is a dramatic saga about the merger. However for brevity this was how a majority party was born that was dubbed the ‘1955 system’ by the media and political analysts. The LDP at the time had the profile of conservative center right leaning and contrasted rather well against the ‘leftists’ (communists and socialists) leaning opposition. Ideology is not necessarily important. The opposition left was big, but weren’t always united. There is also a history of ‘leftist’ disfavor due to the Cold War by the American Occupation. See Dower (1999) for more. There might be some influence at that point that caused subsequent decline of the ‘leftists’ from their heights in the immediate post-war, but also other things like changes in material conditions might also explain their decline. So the LDP had a favorable start.

So the electoral system is an important factor. First, in 1906 an electoral system dubbed the Single Non-Transferable Vote (SNTV) was instituted. This continued into the post-war until 1994 when a new electoral was implemented. So electoral districts were drawn up like the UK House of Commons or the US House of Representatives. The election scheme however allotted districts to have multiple seats unlike the UK or the US who are single seat districts. So there were three to five seats per district. So for instance a district with 5 seats with 500k voters elects 5 representatives even if each got equal number of votes of 100k. In a lopsided election where say one candidate got 460k and the remaining 4 got 10k each, this would still elect the other four candidates. (Consider the variety of combinations). So this is what non-transferable vote means because the votes of the dominant candidate weren’t as powerful as the 10k votes received by the others. So the incentive was to create a base of voters to get a substantial number of votes to be elected. So this is what might be called ‘clientelism’ or personal politics. So this then introduced ‘money politics’ or ‘vote-buying’. So early on this was identified because of in-fighting and party policy fragmentation beginning in 1956. Political parties could field multiple candidates in a district as there was nothing to stop that. Same party candidates would fight against each other in addition to the opposition. There is also a slow saga of electoral and district reform. The LDP is not fully united as there are factions within the party. The factions are almost like their own political parties so it was/is like herding cats because of the varying incentives for political power. Yet there was an effort to create more ‘normal’ governance structures incentivized by the wider electorate, but obviously this is politics and no one wants to gives up power for uncertainty.

Scheiner (2006) makes the argument that with clientelism, then fiscal centrality and bureaucratic protection of the clientelistic networks were a means for the LDP to maintain a political base in the countryside. Fiscal centrality as mentioned earlier created limited local autonomy and thus more dependence on the central government. The post-war economic development created winners and losers so the rural areas lagged due to urban favoritism in the manufacturing and service sectors. Budgets constrained needed to be supplemented by the central government. So the rural populations had quite a lot of electoral power due to foot dragging electoral redistricting reform and the rapid change in population distribution in the post-war. The bureaucracy was very much leaning toward the LDP due to the history of LDP politicians managing the bureaucracy. See Nobusuke Kishida, Shigeru Yoshida, or Hayato Ikeda. The bureaucracy plays a very powerful role in Japan and it is the way that governance was set up structurally and institutionally that make it more powerful than even the politicians. Add that big business was recruited as public policy apparatuses created what Japan analysts call the ‘Iron Triangle’ or ‘Japan Inc.’ The situation created dependencies and feedback incentive loops that contrast liberal democratic governance found in Europe or the US.

So the bureaucracy very much helped the LDP as there were ‘carrots and sticks’ to create networks of support. The bureaucracy in Japan is a complicated topic and just know that it is very different. In the US, bureaucrats are bound by law and heavy regulation like the ‘oath to uphold the Constitution’, but in Japan there is a heavy contrast and the law was vague enough to allow a lot of power in the bureaucracy with varying incentives. The brief and simplified history is that bureaucracy was sort of the representation of the emperor as it was originally set-up in the Meiji and subsequent eras. This could explain a bit of how democracy declined in the 1930s to get Japan into its empire fiasco. Yet the US Occupation used the very same bureaucracies and their machinations to manage Japan with just minor changes. So there was continuity and evolution of bureaucratic power. The bureaucracy would use its levers to reward and punish localities in purpose to create stronger incumbency for the LDP. This could be distributing funds, rigging project bidding for favored companies, and just plethora of things to favor the LDP’s incumbency. In a way the bureaucratic is a faction within the LDP.

So in addition it might be spoken a bit of how the economic recovery and subsequent economic growth created political inertia for the LDP. Probably the most important prime minister was Liberal Party’s Shigeru Yoshida who in his second term as PM negotiated San Francisco Treaty which gave Japan back its sovereignty and ended the American Occupation. He also created what would be called the ‘Yoshida doctrine’ which was the policy of using Japan’s relations and status especially with the US as a means for economic development. So there were negotiations of treaties, agreements, and arrangements, but Yoshida kept an eye on what Japan could gain via trade or other advantages. The history of Anpō (the US-Japan Security Treaties) was one example and the negotiation of Japan’s membership to the GATT (General Agreement on Tariffs and Trade precursor to the WTO - World Trade Organization) is another. Domestically this played out as quasi-corporatism or economic nationalistic policies such embodied in the ‘Iron Triangle’ where the state tried its best to benefit Japanese businesses and politics. The main point in this paragraph might be that things were not so bad (actually good) to cause radical changes in allegiance away from the LDP. The LDP invited the material changes and economic development to shore that clientelistic base. Without development, there can be no riches to spread.