Why has the US Census become less productive over time?

From a blog post, found via Marginal Revolution:

In the US, the number of census takers, or “enumerators,” has risen faster than the population: from 1940 to 1970, there was slightly less than one enumerator per thousand people counted, but in 2000 and 2010, the intensity of census employment was just below two enumerators per thousand. The rise in the intensity of US census employment since 1970 is quite surprising because the census has steadily introduced labor-saving technology during that period.

In 1950, census employees knocked on every door and conducted an interview. If no one was at home, the census taker had to keep coming back until the interview was completed. Other census employees in Washington and Philadelphia then spent the summer and fall months transcribing the surveys onto computer-readable punch cards. The whole process was demanding and inefficient. [...]

For this reason, in 1960, advance surveys were delivered by mail, and census takers could simply pick up completed surveys, yielding a major time savings. Census employees then transcribed the questionnaire by hand onto another sheet that could be read by FOSDIC (Film Optical Sensing Device for Input to Computers).

For the 1970 and subsequent censuses, the Postal Service took on an even greater role. Most households received a machine-readable survey by mail and returned it the same way. This cut out two labor- intensive processes: canvassing most households and transcribing data by hand. Censuses since 1970 have generally followed the same process. The biggest change was that the 2010 survey dropped the “long-form” census – a major labor-saving change that nevertheless did not have an obvious impact on the amount of labor expended.

(No definitive answer given, just some possibilities.)