[addicted]

The answer is funding.

USCIS needs to be fully self reliant on funding, a restriction not placed on any other agency from what I know.

Congress passed a bill a couple of years ago increasing the fees of several kinds of applications allowing for more funding, after a long gap. Further, they passed funding for digitizing their processes and updating computers after about 2 decades.

An additional factor is the massive backlog created by the Trump administration whose strategy was to simply add roadblocks and delays to the simplest processes. This was further exacerbated by pandemic slowdowns, so the increased capacity, combined with the removal of ideological opposition to legal and authorized immigration and a large backlog has likely resulted in the large numbers of citizenships.