What broke with advertising was its disconnection from community, just as with publishers.
We Can Fix This F*cking Mess.
John Battelle

So, blame DoubleClick (now part of Google).

Of course, publishers could try selling their own ads, and that’s exactly what the most successful niche sites do (Wirecutter springs to mind). But that gets difficult at scale because what advertiser wants to deal with 3,000 different publishers?