My first reaction would be that COBOL would run better on COBOL-grade systems.
Snarkiness aside, there is some truth to that remark. Our school computer in the late 70's was an IBM S/370 mainframe. It used data types that I have not seen anywhere else, like packed decimal which was handled by the CPU's instruction set. If the IRS' COBOL programs used something like packed decimal and newer computers don't support packed decimal, then the tried-and-true legacy software would not work. Therefore, they needed to keep the old computers running.
At work (embedded engineering), we had many utilities and other software tools that no longer work because 64-bit Windows no longer supports 16-bit applications. Our solution was to keep Windows XP, but then we migrated to setting up virtual computer emulators to run on our new computers.
Similarly, I wrote my program for calculating the probabilities of my MONKEY in Turbo Pascal whose extended REAL data type I used. Now I cannot find support for that extended REAL data type, so I cannot run that program with anywhere close to the same precision. I would need to set up a legacy system with legacy software.
The computers is only part of the story. The other part is reduced staffing.
One reason for reduced IRS staffing is the pandemic and the need to provide IRS agents proper social distancing. In order to provide that, the IRS simply had fewer agents working in the same office space. And trying to do the same amount of work as a larger staff used to. Which they couldn't.
Last year I had to get a tax number for our parents' trust. What should have taken only a couple of weeks ended up taking nearly two months. When I was finally able to get through to a person (that journey alone can yield hours of woeful tales), she told me that the reason it was taking so long was understaffing and that the understaffing was due to COVID social distancing requirements.
The other reason is that the Trump Administration cut the IRS' budget. Part of the reason for that was to hamstring their ability to do their work, which included investigating tax fraud (AKA auditing) and collecting on overdue taxes, etc. That was also part of Trump's hollowing out of government agencies in order to destroy them (and their ability to enforce regulations), which makes the next part all the more interesting. At the same time that Trump wasn't making any permanent appointments but rather all agencies, including DoD, were being headed by acting directors, Trump made very sure to appoint his flunkies to the top three positions at the IRS; not acting heads, but rather actual appointments. His goal in that was to have them block any attempts to have his tax returns released.