Over the years, marketing changes have made it a lot harder. As publishers, we like to think that AdSense "has our back", but the fact is that a lot of changes made in AdSense's products are usually precipitated by advertiser satisfaction, rather than by publisher's demands. And because advertisers can now place ads for much less expense, and there are many more publishers and monetizable platforms, the earnings are driven downwards on a CPC basis.
In everyday terms, it's representative of the supply and demand theory. When you have a lot of something (in this case publishers), and less demand for that something (not as many advertisers), the cost (in this case CPC) should go down. But supply and demand is not quite as simple as that. Even if you have a lot of publishers/websites, if you also have a lot of advertisers, the cost should remain relatively stable.
Then there are the advertisers. While the advertisers might not be equal to the numbers of sites using AdSense on them, the advertiser numbers grow and decrease monthly too. New advertisers and business owners spend their advertising budget with AdWords; some leave, or close their businesses, sell their websites or ... simply run out of money.
As the internet grew, so did interest in online advertising grow. Today, there are hundreds of established companies that provide online advertising to businesses and individuals and the explosion of ad agencies also helps to drive the cost to advertise "netwide" (ie: not just AdWords cost, but advertising across the entire internet) lower.
That leaves publishers with lower earnings per click than what they might have made regularly back in 2003. Long term publishers will have experienced decreased revenues on a CPC basis fairly regularly over the years, so while it hasn't been a lot of fun to deal with decreased revenues, it is something they've learned to handle. Publishers that learn to expand their holdings into other platforms (like apps and videos) can sometimes increase their earned revenue by making use of new technology, even when the CPC rates stay the same.
Publishers who have less experience with AdSense tend not to be quite as stoic, and are often less inclined to use new platforms.
We've seen lots of frustration, anger and bewilderment in the forums over lower earnings. Sure, there can be other reasons for a (a lot of invalid activity/traffic/clicks for example) reduction in earnings, but in the end there is only so much a publisher can do when the lower earnings are based on the cost per click.
Create new, original and interesting content regularly (ie: WRITE YOUR OWN), increase your readership, expand into new platforms and look for realistic ways to increase your earnings (staying within AdSense policy of course) - but no matter what you do, you have to be the "author" of your own success. Nobody else, not even AdSense, can do it for you.
posted by J. Gracey Stinson