Before todays Monster Rampage started CM Mirade jokingly said to not even ask for Apocalypse or Arkhothep and there's no chance for them being summoned. It's most likely due to people constantly asking and shouting names of monsters form this list:
R7 bosses, Arkhotep, Abomination, Zoralurk, Freegoiz.
I have been joining the rampages for almost a decade now and people have been always doing that ever since cip suddenly decided to stop summoning them. For some cases it's understandable, like Abomination became a regular boss after years of being TS exclusive. Seeing that CMs are perfectly aware of huge demand for these unique lore-related bosses I mentioned before it's weird that they just straight up refuse to summon them. Monster Rampage is for fun only event so why won't they try to satisfy their players and give them what they want? Has any cip member ever officially addressed this?