1) it is propagated by scientists on a non-scientific mission
2) it is believed because it plausibly explains an observation (increasing global temperature [for a time], increasing heart attacks from smoking in the 1950s and 60s). It taps into large anxieties about too much wealth, too much happiness, in western societies. There must be sin somewhere, and the public is ready to flog itself in the cause of a secularized idea of God, uh, I mean Good.
3) the causal relationship is weaker than first supposed; the research is found to be sloppy, the facts have been fudged, subsequent studies do not fully support the original claims, nevertheless the orthodoxy is promulgated all the more harshly for being doubted.
4) by now, powerful economic and ideological interests have taken hold. They supply an ongoing source of funds and opinion to ensure the perpetuation of the alarm: in the case of cholesterol, the margarine industry, the pharmaceutical industry, and the medical establishment, and in the case of AGW, the tribe of bureaucrats and leftists who seek to control markets, whose god of Marxism had failed, and who needed a new god (Gaia) to justify their rule.
5) The skeptics who have patiently argued on the basis of facts that the science of each phenomenon was weak, are ostracized by the opinion establishments of medicine and global warming. Cranks, but the cranks are right and the orthodox priests and Levites are wrong.
6) Eventually, after fifty or sixty years, the subject of discussion just changes. In the case of cholesterol, the evidence gets weaker and weaker, and the problems caused by too much sugar consumption (obesity, diabetes), caused in part by people not eating enough fats and meats, reaches a stage where it can no longer be ignored.
7) the retreat of the orthodoxy is covered by a smokescreen of fresh concerns for some other catastrophe. No admissions of error or apologies for wrecked careers and following bad science are ever issued. Time flows on, bringing neither knowledge nor greater understanding of the role of folly in human affairs.
8) stages 6 and 7 have been reached in the cholesterol cycle; they are beginning in the anthropogenic global warming scam. Fifty years from now, there will still be clanking windmills in the North Sea, but whether they will be still linked to a power grid is less likely, and whether anyone will pay attention is doubtful. The lobbies that keep them there, however, will still exist.