It's all about a typical bureaucracy flexing its muscle to get its way, damn the passengers!
Zero Hedge explains:
The TSA has been urging passengers to get to the airport at least two hours early for domestic flights, and three hours early for international flights. However, some passengers have said that is not enough time to get through security and still make their flight.
One alleged reason behind these lines was that Homeland Security wanted to force more travelers to become “known” to the government via PreCheck programs. Another reason was the TSA's revolt at the government cutting its funding. According to AP, "in the past three years, the TSA and Congress cut the number of front-line screeners by 4,622 — or about 10 percent — on expectations that an expedited screening program called PreCheck would speed up the lines. However, not enough people enrolled for TSA to realize the anticipated efficiencies."
In other words, PreCheck has been in a failure. Not enough Americans have been willing to make themselves “known” to FedGov and Homeland Security, and now the epic inefficiency of the TSA - and the government - has been fully exposed.