Blogging as an income or business model became famous basically because of the Google AdSense program which made selling of ads easier. Media companies have sales people who go and sell their ad inventory to companies but individual bloggers don’t have the volume or the time to sell ad inventory directly. But ad networks like AdSense make it damn easy to just put up the codes and make monthly income from their websites.
The income from such a business model is highly volatile and you have no control over the traffic that comes to the website. Most of the traffic usually comes via search engines and everyone knows that they change their algorithms every now and then and then the amount of traffic that comes to the website suddenly changes. The amount of control you have over the income is very low and hence this business model is not as strong. Even if you have very high traffic and the man power to sell direct ads, display ads in general are not very effective and hence it is not a great “product” to sell. If advertisers do not get a direct ROI on their ads they will not keep their ads running.
Many company go for display ads only for branding purposes and they are not consistent. Hence I would say the business of selling display ads is one of the weakest business models online. Big websites are now exploring other opportunities to generate revenue like conducting live events, selling membership tickets etc.