Why hasn't the growth rate of real potential US GDP recovered?

Missionary Work Among Savages aka 2slugbait wrote about whether President Reagan's 1981 tax cuts performed as advertised. I think his motivation was to determine if a similar supply side approach could have been a better alternative than the actual mild fiscal stimulus measures implemented by Bernanke and Obama, immediately following the 2008-2010 financial collapse. In other words, would a Reagan-type tax cut in 2008 have driven a stronger recovery.

Supply side economics maybe not so great

2slugbaits demonstrates empirically why supply side economics was only mildly effective. Specifically, none of the following worked quite as well as Arthur Laffer's forecast:

1. cutting the top marginal tax rate would encourage greater labor effort 
2. supply side structured tax cuts would encourage greater personal saving 
3. the tax cuts would pay for themselves

Skepticism about David Stockman and Laffer's economic policy recommendations during the Reagan Administration is only mildly interesting to me. The correct sort of tax cuts (cuts that include middle- and lower-income wage earners, not solely high net worth "job creators" 😕😦😡) CAN provide a modicum of supply side stimulus. That was apparent from 2017 to March 2020.

Something else is much more interesting to me: the long-term decline in real potential US GDP.

Real Potential Gross Domestic Product

2slugbaits elaborates on item 3 above, noting that:

"If the Reagan tax cuts actually affected the supply side of the macro economy... then we should have observed an unparalleled increase in the growth rate of real potential GDP... Yes, real potential GDP did grow at a pretty good clip immediately after the Reagan recession, but it quickly faded...  even at its peak it was only barely above the growth rates during the Nixon, Ford and Carter years and well below rates enjoyed during the LBJ and Clinton years."

Real Potential Gross Domestic Product is defined by the St. Louis Federal Reserve (FRED) as "the Congressional Budget Office (CBO) estimate of the output the economy would produce with a high rate of use of capital and labor resources." The data is adjusted to remove effects of inflation. CBO measures real potential GDP in non-seasonally adjusted billions of chained 2012 dollars. Frequency is quarterly.

Gold Star of Texas

The state capitol building in Austin, Texas (21 October 2008)

Cupola of the state capitol at Austin

Photograph by TedLandphair via Picasa