Don't forget the impact of the economy in general. The effective rate is dependent on income and during recessions the effective rate decreases as incomes decrease.

This set of data shows the effective rate for the top 1% isn't that much lower than it was in the 1950's (baring the recession in 2008) and much higher than they were in the 1980's when Reagan gave his "tax cuts to the wealthy".

https://files.taxfoundation.org/20170804133536/Average-Effec...