Lee Drutman, co-director of the Legislative Branch Capacity Working Group, writes on Vox:
So what accounts for the Senate's almost 30-year drought of "notable investigations"? The most obvious explanation is that doing major investigations is hard work. It requires considerable staff capacity. But even more than just numbers, it requires experienced staff who have put in the time it takes to really understand how Washington works. It also requires staff who are planning to stick around long enough to see these kinds of investigations to fruition. These kinds of staff members are in shorter and shorter supply on Capitol Hill these days.
Consider some numbers. In 1975, when the Church committee was in operation, the Senate employed 1,277 committee staff, according to the Brookings Institution's Vital Statistics on Congress. In 2015, the Senate had just 888 committee staff, a 30 percent decline. Over these four decades, the demands on Congress have increased, the complexity of the world has increased, and the executive branch has expanded considerably. Yet the Senate has been shedding committee staff during this period. The House has also had a similar decline, from 1,460 committee staff in 1975 to 1,100 in 2015 (down 25 percent).