Not even to fit, necessarily, but also to pay for them. (The BOBW features weren't created until after the separate disc release existed, per interviews on... TrekCore?... with the people behind the new special features.)
Precisely. We'd never have a 30 MINUTE retrospect from BOBW, a dedicated Gag Reel or an Audio Commentary if the single-disc wasn't commissioned. It gave the team a new budget to work with to CREATE new VAM.
Plus - it's a wonderful introduction for people who haven't seen the season sets or can't afford them yet.