The Onion

I'm not exactly sure when it happened, but I've got this sense that the shift coincided with increased public awareness that police were generally being pretty shitty to marginalized and black/brown communities. I think we probably all knew that on some level, but through a mix of generally more racist attitudes that existed back in the 60s/70s/80s and lack of exposure to the problems those communities faced, it was pretty easy for lots of the country to happily go on believing cops were the good guys.

Through the 90s/00s/10s it got increasingly harder to ignore or justify shitty policing tactics, even to those who weren't directly impacted by them. And when the calls for change and reform started to come from more diverse audiences, police unions and propaganda groups decided to circle the wagons and ramp up the us-vs-them attitudes towards the public in general, rather than tackling the problems that were being called out.

There's a whole other level of WHY they chose that path, but that's my 2 cents on the surface level question at least.