I can't speak for each brand or agency, but the majority have been vocal with YouTube or FB or Twitter for years. Each of those ad platforms has a slew of teams that work directly with the enterprise buyers (top advertisers). I remember a major USA bank giving explicit feedback to the YouTube product team six years ago due to limited brand safe content control. I was in the room. There are still two accepted strategies for delivering ads on user generated content networks. One, that if user targeting is used and an ad is served against questionable content, then the content is less important. Why? You delivered an ad to your target market. Two, even with user targeting, the ad's context and content should align with the advertisers ad. I think we are seeing #2 uptick across the board.
For networks like YouTube and Facebook they are in control of the algorithm, feed, ad decisioning, user targeting, and cost. Why can't they afford transparency and more control for advertisers? I don't know to be honest. I assume those features would limit revenue.
To answer your last question, I also don't know what tipped it. It seems like society is shifting quickly right now and brands want to better align with those changes. Someone else asked if all of this was for the sake of practicality. That is a part of it. Users are influenced by advertising and while we can't decipher the exact mix of it that truly influences behavior, brands are trying to protect themselves from egregious content.
I try to create physical analogies for my clients. Let's pretend the path to our storefront (selling clothes) is in a manufacturing warehouse park. Who is going to be enticed to enter the path to the store, or be curious to learn about the store's wares? I predict few folks would enter. It is similar to ads across the web. Some are there to entice you to click (enter) their store and some are there to inform you. Either way, context likely has some influence. Brands acknowledge that. Not to mention, zero brands want someone to screenshot their video or display ad next to something deemed malicious.
Lastly, it is hard for advertisers to turn their backs on channels that generate high returns or are table stakes. What would you do if the billboard that drove the most customers to your store was in a seedy location 20% of the time? The billboard owner sold it as is. So you either take it or leave it. Most business owners would buy it and deal with the risk. That is what we've seen play out on YouTube and Facebook.
Edit: I will add that sophisticated advertisers are shifting their performance metrics from pure lead-generation sales or site visits to incrementality measurement. They want to measure if ad exposure changes consumer behavior. Perhaps we will see that FB and YT have high influence or not or something in the middle. The changes in measurement will have major impacts on advertising spend.