Not surprisingly, uBlock Origin relies heavily on features in V2 to perform it's functionality. It could probably be rewritten to use V3 but it's not a simple "Hey ChatGPT, make all this code into V3 compliant code." It will require a pretty fundamental rewrite.
Google made common sense improvements to the way extension work for safety and performance. Were they sad that uBlock Origin stopped working? Probably not. But I highly doubt it factored into the need for the V3 changes.