Why has the time spent studying declined so sharply in the United States over the the past few decades? I read on https://www.aei.org/publication/leisure-college-usa/ :


Study time for full-time students at four-year colleges in the United States fell from twenty-four hours per week in 1961 to fourteen hours per week in 2003.

The authors give some inconclusive statement regarding the reasons behind the trend:

The decline is not explained by changes over time in student work status, parental education, major choice, or the type of institution students attended.
Evidence that declines in study time result from improvements in education technology is slim. A more plausible explanation is that achievement standards have fallen.

I wonder why the time spent studying has declined so sharply in the United States over the the past few decades. I am only looking for data-supported answers (or pointers to them).
First, it is important to deploy your scientific skepticism in assessing this claim.  The source, after all, is the American Enterprise Institute, which is a political "think tank" that is explicitly dedicated to pushing a particular point of view.  Other key elements of its scientific record including taking tobacco company money to produce pro-smoking studies the 1980s and recent attacks on global warming.  

There does, however, seem to be at least some difference there, and a deeper analysis of both the data and possible causes found that a major transition happened across the 1970s:


Expansion of the student population to include many more people who are working to support themselves, parenting, commuting long distances, etc., which means they cannot devote as much time, and
A shift in faculty requirements away from teaching and toward research, with a concomitant decrease in the amount of work that faculty are able to support asking from students.


From the early 1980s to today, however, the situation appears to have been more stable.