Generally I blame Congress for the pile of work dumped into the IRS's lap the past several years.  ACA, TCJA and everything COVID.  But then the IRS brings a good amount of it on themselves when they do things like this:

https://www.irs.gov/newsroom/irs-sending-letters-to-over-9-million-potentially-eligible-families-who...

When you can't answer the phone and it takes 6 months to open the mail, you don't need to be drumming up new business.