"BRAND SAFETY"
No well known advertiser wants their brand associated with something unpopular or toxic. This is why YouTube is demonetising any video that mentions Covid-19. YouTube themselves technically probably don't care, but brand safety conscious advertisers certainly do, and so YouTube must too.
They've realised that Twitter and FB, and others, are currently toxic platforms. The discussion threads can be awful, driven by fascists on both the far-left and far-right of the political debate. They no longer want to buy this advertising space, it's too much of a reputational risk (I will say that advertisers outsource the buying of advertising space to Ad agencies who may not be as savvy since they're under pressure to spend allotted advertising budgets).
Hence the news about big brands pulling budgets from social media.
Another issue that I've never seen discussed is that if Twitter (and other social media platforms) relies completely on advertising revenue, it can be forced to only allow content that advertisers approve of. In other words, advertisers can in theory control political debate. Social media platforms then ban or remove content that's undesirable for advertisers because they can't monetise it.