I found this archived news post detailing the change from Eclipse to Astral Power -
Balance Druids have had a variety of cyclical rotations over the years. The core design problem in all of these cycles is that they’ve forced players into watching closely for UI cues and casting spells in largely predetermined orders. The Legion design is focused on providing a flexible combat experience, where you can use abilities as you’d like, and each has a specific purpose.
I miss this design. Why go back to an awful, restrictive rotation? Layering systems is cool if done correctly, but this current version of Eclipse fragments and gates damage far too much.