BMB was discontinued after its only release because it was, in part, a massive testbed for a ton of experimental items and mechanics. Reintroducing the mode would be a matter of deciding whether to either reintroduce it in the exact same form as it once was, or whether to revamp it and introduce new items and mechanics. The latter would be particularly difficult to do, because it would mean BMB would essentially need to be turned into a new game mode with every release.
By contrast, Dark Star is a game mode that is completely independent from pretty much anything else in the game. Literally the only thing it depends on is Thresh, so unless he somehow needs to be turned into a totally different champion (doubtful), Dark Star can be introduced at any time without having to worry about changes in champion roster, itemization, summoner spells, runes, masteries, etc. In that respect, it's probably one of the easiest modes to have in RGMQ, since you can be pretty sure it's not going to need adjustments relative to the main game.