Region, game mode, and even "tier" (Recruit Conditioning On/Off). There is at least 48 discrete player pools with no simplified way to browse between active matches or at least see if there are full lobbies to queue.
On top of it, ping limiter can prevent you from joining active matches with available spots and there is also a random bug that gives the "session full" error even when joining to a lobby with 2 to 7 players (out of 8) from friends list.
It depends of the time of the day, but yeah.
NA or EU in their respective afternoons and evenings.
They never remove