For the same reason popup blockers became a thing. Who doesn’t remember the hundreds of windows that would open. Thousands of dialog boxes which would prevent you from closing the windows and so on.
I have zero sympathy for ads. They messed up bad and there is no way to recover or regain any trust.
It feels like you can join in later. There are not thousands of replies in the first few hours. So commenting or participating was a waste of time before in many bigger subs. Noone would ever see your answer anyway or interact with you, so there was really no point.
Here it feels like you are actually participating in some way. I really like it.