My guess is staffing, or poorly planned staffing levels.
I had an instance where C had no PC on a Sunday afternoon, so I walked to B to find the same situation. This was in January (I think). Queue was short and I got PC Lite (WTMD, shoes and belts stay on, laptops and freedom baggies out). Added maybe 10 minutes to the security experience, mostly because of the shoe carnival ahead of me.
Mrs. pa3lsvt was flying last Tuesday afternoon. B had no PC so she walked to C and all was well.
I clear through D/E quite often when I have to park in the D/E/F garages and have yet to find a time when PC was closed. Can't recall the earliest I have arrived at that checkpoint.
It seems that the TSA at PHL is taking a page from the US ground staff management playbook - mediocrity is sufficient.