It's 'Alarming': Children Are Severely Behind in Reading
nytimes.com
nytimes.com
The pandemic affected everybody almost equally by political jurisdiction so demographic stratification suggests there is more to blame in addition to the pandemic. Looking at young children in my area my first blame is convenient access to touchscreens.
If by "the pandemic" you are referring to things like the closures of schools and businesses, then this is certainly not the case.
I would imagine the same basic things are playing out here in these stats as well - the ability to have good reliable internet, a dedicated computer or tablet to work on, a proper physical space or desk space to do the work, etc. All of these things tend to be a given with higher income levels, but are not at all a forgone conclusion with lower income levels.
Some good homeschooling stats for 2021: https://www.thinkimpact.com/homeschooling-statistics/