It doesn't matter for most devs, unless you don't support the game anymore, this doesn't help anything, at some point you will need an engine update to support new hardware, fix an engine bug or similar.

Any idea of why Unity did this?

I mean, they'll generate some short term cash, sure, but they just lost their entire customer base. No developer of any size can take on the liability and risk of working with Unity again, even if Unity realizes how badly they screwed this up and reverts this.