In the case of the Motor Boat Cruise and the Skyway their operation costs were diverted to Mickey's Toontown and Indiana Jones respectively. Mike Fink was deemed too dangerous (and costly) to operate.And none of those are major attractions either. I do still miss the People Movers and Skyway though. And most of these just became outdated. Can you imagine having Flight to the Moon or Adventures through Inner Space today? They were fun for their time, but today, they would feel like relics.
And my guess is most of these just saw falling attendance.