They may just not care about being the game engine of choice for indie gamedevs anymore, or they may even actively want to not be that anymore (as a branding thing). The way Unity positions itself on their landing page is VERY different from what they were doing 5 or 10 years ago, when (indie) games were absolutely front and center (and there wasn't much else, period).

I think being a AA game engine was just one early part of their long term business strategy, and this last move is them ejecting that stage of their rocket as a necessary part of moving on to the next stage of their strategy.