The main issue with it being an 8-person event, I'd figure, have to do with the logistics of party matching. Making it 4 players in practice makes it very easy for them to continue to insert it into currently existant roulettes, instead of needing to separate it off the way Castrum and Praetorium are. And I doubt the Castrum/Praetorium roulette as is would be enhanced by forcing people used to high level play to stomach "oh boy I have to pull noobs through this" - if anything, it'd aggravate the problems already present in Main Story Roulette's existence.

And as mentioned, for a full blown dungeon, >4 players starts becoming trickier to design content for in a meaningful way. There's a reason Coil, CT, etc. have very, very little trash.