At least with a print paper you were reasonably certain the ads were reaching people in a certain geographical location (and you could data mine your subscriber list for more stats).

With an online site -- especially one with no registration required -- you know very little about the people reading, so advertisers only pay commodity rates for ads that could apply to just about anyone (whiten your teeth, refinance your mortgage, etc). Advertisers are still very willing to pay to reach potential customers, but newspapers in the online world haven't really figured out a way to connect to them.