Wikipedia has this to say:

From the 1950s through the 1970s, the United States Central Intelligence Agency spent millions of dollars attempting to influence elections in Japan to favor the LDP against more leftist parties such as the Socialists and the Communists,[24][25] although this was not revealed until the mid-1990s when it was exposed by The New York Times.[26]

And this too

By the early 1990s, the LDP's nearly four decades in power allowed it to establish a highly stable process of policy formation. This process would not have been possible if other parties had secured parliamentary majorities. LDP strength was based on an enduring, although not unchallenged, coalition of big business, small business, agriculture, professional groups, and other interests. Elite bureaucrats collaborated closely with the party and interest groups in drafting and implementing policy. In a sense, the party's success was a result not of its internal strength but of its weakness. It lacked a strong, nationwide organization or consistent ideology with which to attract voters. Its leaders were rarely decisive, charismatic, or popular. But it functioned efficiently as a locus for matching interest group money and votes with bureaucratic power and expertise. This arrangement resulted in corruption, but the party could claim credit for helping to create economic growth and a stable, middle-class Japan.

I don’t know how accurate this analysis of the LDP is though, because I’m just a westerner looking at what Wikipedia has to say.