#1 - The internet moves and changes faster than most people can keep up. #2 - Google will 'Target' ads to signed in users...so you could have a health blog but less targeted ads could roll 'with a lower epc' than your typical rate. #3 - There's little competition. When you're the only game in town, its tough for anyone to put pressure on Google for change when there is nowhere else to go if you want widely used PPC ads running on your site. #4 - Could be your traffic. Different sources (even at higher numbers) may not convert as well as others.
There are lots of PPC networks out there, but none on the level to compete worldwide with Google; by comparison, they're (competition) like feeding a tic-tac to a whale.
PPC is by far the most affordable form of advertising available online. It has been widely used by most of the internet marketers today. It is a creative and effective to promote products and business services.