I can think of a couple of reasons the seat may have remained empty - both happened to me in years past (though not recently).

1) The seat was broken (mosf often, the seatback won't stay in the upright position). This of course is a FAA rule. I was once upgraded and when I got to my seat, the back was like a broken recliner. The FA told me I couldn't use the seat so back to coach I went. They handled it professionally and offered free booze and headset (I was taking a prescription so could not accept the offer).

2) Not enough FC meals banked. This happend too often in the 1994 bankruptcy days, but more recently the computers track meal banking very well. For example, say you're flying on a 12-seat-FC plane; 9 seats are "sold" but YM has held back the remaining seats, so it's A3 F0. The computer "knows" that there are enough elites to eventually be moved up into those three seats, so it will order a full load of FC meals. But I'm sure breakdowns in the procedure happen.

I'm not 100% sure but I think CO has (had?) a policy that they will not place a passenger (rev or otherwise) in FC if an FC meal is not available "to protect the integrity of the service presentation level" or something like that.