Reading comments, I think many still don’t get what “beta” means. It’s not a privilege, you’re testing unfinished software with bugs to be fixed.
Other brands like Samsung can afford to send betas to flagships first because they are confident that they have already sold tens of millions of units worldwide, so there is a large sample among which to test the software and get enough feedback.
Nothing needs to make sure that the beta reaches as many people as possible to get all the necessary feedback and polish Nothing OS 3. So, logically, it sends it to its most popular device first.
