The IRS recalculates the sales tax tables each year based on statistical information about average spending habits in each income range. Many factors could influence spending habits and cause the average sales tax to increase or decrease. If you compare the 2019 and 2020 sales tax tables for your state, you will probably find that most of the figures are lower for 2020. (The sales tax tables are in the IRS instructions for Schedule A.)
We can guess at some of the factors that might have caused a decrease in the amount of sales tax that people paid, but understand that the IRS is not analyzing the causes. They are basically surveying how much sales tax is paid by people at various income levels, without necessarily knowing the reasons. One possibility is that overall spending on taxable items decreased in 2020 because of the Covid-19 pandemic. Another possibility is that some items that were subject to sales tax i 2019 were made nontaxable in 2020. I'm sure there are many other possible causes of the decrease.
