bidvertiser and other lesser names as opposed to adsense or YPN? Do they have some special deals or something?
If you think about alternatives to Adsense a lot of them offer better deals, not just to national newspapers, but practically to anyone. Thus, it doesn't make much sense for a newspaper that gets about 50,000 hits a day to loose extra $5000 by going through adsense and not a smaller site. I would chose those over google myself, except for beginners adsense is more convinient.
It can also be simple marketing. The big guys will simply take a use us or not attitude. They might give special deals, but nothing major. However, a smaller guy will bend over backwards, because just having their advertising on the site builds credibility. Therefore, if it means operating at a loss, it can still yield positive results.