I can only speculate of course, but I'd suspect a far simpler reason, driving subscriptions. Even limited use for video editing can fill up the free 5GB tier very quickly and if iClouds rather aggressive nudges to upgrade and their success amongst the people around me are anything to go by, there is a lot of potential in getting people to subscribe for a few bucks.

The scenario I envision is basically, a user just edits a few 4k files together and starts approaching the 5GB limit. Because OneDrive is also by default used for other things such as documents and even the desktop nowadays, they soon thereafter hit the 5GB limit and start being inundated with offers to upgrade to the 100GB tier. "Just 1,99 per month, nothing major, barely felt", at least that's the pitch. Maybe they acquiesce, maybe they ignore it. That's the play I'd wager. And if enough people resist, maybe in an upcoming update a full OneDrive could lead to (artificially) degraded functionality. Not in the sense of local storage being impossible, just slightly less convenient over direct OneDrive. Say what you want about the iCloud nagging (I have and will continue complaining about that as well), at least it isn't required to have free iCloud storage to locally edit ones videos with iMovie.

In any case, forcing OneDrive for a video editor (arguably one of the highest storage requirement programs most people will ever use at the moment) is anti-consumer and showcases how little any commitment [0] by them actually means. Took less than a week, which is honestly longer than I'd have suspected...

[0] https://blogs.windows.com/windows-insider/2026/03/20/our-com...