I always understood falling marriage as being primarily linked to the rise in women's education and economic independence.

BTW, the total marriage rate by year is a metric that can be easily confounded by tempo effects: if in a country all people born until 1950 married at 20 and all people born since 1951 married at 30, the marriage rate between 1971 to 1980 would be exactly 0 but (neglecting the mortality of twentysomething) no cohort would be any less likely to ever get married than another.