AIUI, the same reason we eventually got "Mary Poppins Returns" after thirty years of trying:
When the Eisner regime first arrived, they were seen as "corporate usurpers", and it was of the utmost corporate-image urgency that they sequelize a Walt-era film to make it look as if Ron & Walt had given their blessing and the torch had been passed.
Unfortunately--oh, did I mention these were simpler times?--they didn't have any idea which happily-ever-after Disney movies
were open-ended enough to sequelize, beyond "Fantasia", "The Rescuers", and "Mary Poppins".
Just how the idea of permanently George-Lucas'ing Fantasia into "Fantasia Revisited" (remember when everyone bought the '90 VHS in a frenzy thinking it really was "the very last time"?) later became the safely separate "Fantasia 2000" is a long story, and the dozen or so Mary Poppins ideas are a thread in themselves, but at least Roy Disney stuck up for the Rescuers sequel.
(Or at least took it up to keep the Animation division from being folded.)