PW is an advertising platform, not a link exchange or some form of webcomics welfare. No one has any obligation to buy any ads on PW at all. Certainly, there’s no requirement that a webcomic site running a PW ad box also bid on PW ad boxes on other sites.
Why do higher traffic sites running PW ad boxes get higher bids (and probably more important to consistently higher bids — get ads from non-webcomic advertisers)? Because they’ve got more traffic and more people reading those ads. Some webcomics also offer the double whammy of a large AND FANATICAL audience. As an advertiser you pay for results. If the result, for example, is getting someone to come check out your webcomic then although small sites ad box prices may be cheaper, the price at large sites, measured in terms of actual person coming to check out your website, is often going to be cheaper still.
Does PW need to attract new non-webcomic advertisers (and publishers for that matter) to grow? Absolutely. There are many ways that could happen and I hope it’s on PW owner Ryan North’s list. But even when that happens it’s probably not going to change the way small webcomic sites experience PW.
As a publisher selling ad space (what you are when you add a PW ad box to your site) be realistic about what type and size audience you have to offer. As an advertiser (what you are when you buy a PW ad somewhere else) be realistic about what you want to achieve with your ad dollars. Plan out ahead of time what your marketing goals are and then focus on achieving them. That may include PW ads, but it may not. There are many, many ways to market a webcomic.