Michiru and Kaoru becoming Cure Bright and Cure Windy was not a normal ability of theirs, but a one-time move made possible by their bonds with Saki and Mai.
Thus why they've been absent from later teamups. Michiru and Kaoru were originally humans.
When the other members of Dark Fall died, they returned to their original forms — even Moerumba, whose original form was heat, was able to shape glass into his image before finally expiring to show what he'd been. However, the twins stayed the same every time they died or nearly died, except when the spirits revived them (twice) in the ending arc, when their eyes stopped looking creepy and they started to look more human. Could that have been their original form? If so, it would explain why Akudaikaan and Gouyaan didn't just kill them when they rebelled, but instead sank them to the bottom of the ocean. If they were to return to their original form as humans who didn't need the power of destruction and ran on a normal life force, that would have been spectacularly counterproductive. Michiru and Kaoru were supposed to be Cures to begin with.
The transformation speeches of this series has references to the one that is supposed to take that transformation. Cures Bloom and Egret transformation speeches references Saki's and Mai's names. Cures Bright and Windy on the other hand, references Michiru and Kaoru respectively.Further, the creators was aiming for a 100-episode runtime just like its predecessor and its successor. The original plan was to have Michiru and Kaoru defect from the dark side and join the Cures for the second year just like Hikari and Kurumi. However this season was cut short, forcing the creators to retrofit Bright and Windy onto Saki and Mai.
The creators finally pulled this off two years later, when Setsuna Higashi defected from Labyrinth and became Cure Passion.
