Not every web page has an associated strong commercial intent. Most sites, quality sites, would lose out to commerical junk sites if only commercial sites could make money from ads.

Can you think of an example where there isn't an associated industry that would be able to serve ads?

A philosophy blog is the first thing that comes to find but even then that can have ads for universities, textbooks, quirky t-shirts, etc.

While you can often come up with relevant tie-ins for websites (a weather site could sell vacation packages, a news site could sell... insurance?) it turns out that for a huge fraction of websites relevant commercial interest is just very low.