I keep comparing it to why Disney thought Fantasia 2000 had "flopped" when it underperformed hitting the suburban cineplexes in summer '00:
The IMAX-only (yes, capitals, dammit) Jan-Apr. '00 release had done sold-out business, and everyone who WANTED to see the movie beat a path to the door of whatever major city had a commercial IMAX screen back then. (Boston didn't have a non-museum one, the nearest was Providence RI, and I wouldn't have seen it at all except for a NYC meeting.)
When the movie finally got its mainstream wide release, there were almost no ads, no press reviews--critics had already reviewed it in January--and those who didn't already hate the concept of Disney sequels either went to see it on brand name, or had some existing IMAX fan drag them to it.
And yes, I'm guessing regular theatrical releases are contractual, to keep them from being classified as Direct-to-Video, and all the regulations that status has at D/P from the old Lasseter-vs.-Tinkerbell days.