I hate to seem condescending, but the "why" is basically what the essay is about for the first 75%. The issue that advertisements have is that the incentives and desires of advertisers usually misaligns with users, sometimes greatly. It's not a concept that's particularly new; for example, take Wikipedia's reasoning for not accepting advertisements. (I'd like to ignore the unrelated issues with Wikipedia funding here, since it's not relevant to this particular point.)
https://en.wikipedia.org/wiki/Wikipedia:Funding_Wikipedia_th...
I don't believe direct sales fixes everything. There's no single party whose ultimate wishes are fully aligned with what's best for everyone. I do think, however, that direct sales and donations are preferable to ads in this regard.