As note multiple times above the priority line is only for business and first no status. I'd tend to believe that it gets shut down when there isn't a lot of demand. I've had it every time starting around 3pm when most of the international flights start checking in and it wouldn't surprise me that they close it down when things slow down later in the evening. I had to go get something around noon from there and it really wasn't needed since there almost nobody heading through Security and Customs.

Side observation: When I've been in Premium Economy I've had to go through the normal line (luckily with Face Connect) and the staff sorting people to the various Security lines told me to head down to the one for the priority line.