Umbara arc barely has Anakin in it.
Yes but they constantly talk about Anakin and how great he is. Also Umbara makes you feel for the clones. Why can’t it be a subplot while anakin is searching for Ashoka
Too much screentime, and screentime is at a premium when you’re editing 6 seasons down into three 2-hour movies.
Look, the Umbara arc is great, possibly the best of the series, but it doesn’t fit if you’re trying to make a trilogy between II and III that sets up Obi-Wan and Anakin’s relationship, Anakin’s distrust of the Jedi and jealousy around Padme, and explains important things not elaborated on in the films like Sifo-Dyas and Force ghosts.
If the idea were to do more than three movies? Sure, Umbara would be a good choice. But there’s just not enough time to include something that’s just a distraction from the primary goals of the edit(s).