This is a topic of much debate amongst economists. Part of the problem is that there is no necessary reason why the things that are important to economic growth should therefore be easily measurable.

In the 1970s, there was a common view that the productivity slowdown was caused by interventionist government ownership of the 1950s and 1960s, and growing inflation rates, but the economic reforms of the 1970s-90s didn't fix this (though the economist Scott Sumner argues in a blog post that the countries that reformed did mostly see productivity increases relative to the non-reformers, excluding NZ). Another argument is that increased rates of regulation, e.g. environmental, health and safety, financial, have raised costs and reduced innovation, empirical testing hasn't found a relationship but it's hard to measure quality of regulation.

The theory that I'm inclined to is that there were a number of inventions and improvements in human capital in the earlier 20th century that combined to drive up productivity, and then they played out. These include electrification, telecommunications, the internal combustion engine, interstate motorways, vaccines, and the spread of primary and then secondary and university education (as those more educated as kids grew to a larger and larger share of workforce). Since the 1970s we have of course seen the internet, but the economist Tyler Cowan argues that the internet by itself just isn't as important as the combination of improvements earlier available. But this theory is also hard to test as it's hard to quantify these factors.

So in short we don't know due to measurement problems.

Sources

Cowen, Tyler The Great Stagnation: How America Ate All the Low-Hanging Fruit of Modern History, Got Sick, and Will(Eventually) Feel Better Dutton, 9 June 2011. ISBN 0525952713

Scott Sumner, 2010, The Unacknowledged Success of Neoliberalism, https://www.econlib.org/library/Columns/y2010/Sumnerneoliberalism.html