They’re likely adjusting the availability of events based on overall player engagement, as well as an attempt to optimise matchmaking behaviours.
When you have more lobbies open, you have fewer average players per lobby, which causes bigger skill/safety gaps between players.
With PDXCE, they seem to have taken the 5 least popular classes and put them on a daily rotation, instead of having them take up a full week slot.
B, A, S and R are all popular enough to sustain a week-long series, but probably not popular enough for multiple series to be up at the same time.
tldr: the game probably doesn’t have enough active players in Featured Multiplayer to support “all classes all the time”
Give this a vote, it’s a potential solution: Searchable public custom lobbies (like FM4)