It's been reported that TSA procedures require a minimum number of screeners to work each lane (due to the positions required to be staffed). If that airport's minimum number is 5 or more (or perhaps if each lane requires a lead Screener or Supervisor that isn't present at the time), then a new lane can't be opened.

TSA also has positions at "Exit lane," to watch for folks coming in through the outbound lane of the terminal. Is it possible one or more of the screeners you saw were at this post? Just wondering.