Offline advertisers pay to have people view their advertisement , even if a few people watch or read the advertisements the advertiser has to pay the amount specified.
Online at present most cpm ad networks are cheating some online publishers, especially the publishers with low traffic websites
If a website has less than 300 visitors in a day , some ad networks are not paying anything to the online publisher despite showing their ads for free.

One ad network is justifying the non-payment to the publisher using the following message :

Unfortunately, either the demand for your traffic is low, or you don't have enough traffic for a stable income.

To increase your income, please, check your ad unit settings and fix them if necessary. Additionally, you can promote your site to attract more quality traffic - this is crucial for high income.

The CPM ad network is charging the advertiser for each page view, so where does the income for the ads displayed on the low traffic websites go? To get even one visitor to a website,the website owner has to spend a lot of time and money in creating content,paying domain, web hosting and other expenses. If the advertising networks are not paying low traffic websites for the advertising they display, the publisher is forced to let the domain expire.

If anyone else is facing this problem of zero income from their websites despite showing display advertising for hundreds or thousands of page views, please share your experience in the comments below.