Computers and improvements in data science/machine learning are basically the entire explanation. A LOT of the techniques that we use today to de-anonymize data require computation power not previously available. Even when doable, resources limited scale. Source: statistics degree

(Also, linkage. There are more data sources to cross reference now with the internet and social media and web tracking and hacks - the record footprint of Americans even as recently as the 70s and 80s was dramatically lower!)