RWW reasons why it is getting tough for new bloggers to make money from Google Ads:

Because of the power law, the long tail of the blogosphere is huge and so any individual blog is not easily discovered. That is, the chance that a random Internet surfer will find a blog that is part of the long tail is nearly zero.

Whatever monetization means the blogger in the long tail settled on, be it Google AdSense or Amazon affiliate codes, it can only work on large volumes of traffic. AdSense works for Google because the odds are in its favor - it is aggregating small amounts of traffic across the entire web. The math works for them because it is based on the massive scale of the web. It similarly works reasonably well for the sites with large amounts of traffic, but it fails for smaller publishers who have low visitor counts. Link.