...Because Adsense Sucks. It's not worth the Real Estate. Adsense is for Feeds and footers...it's not worth it when there are so many well paying programs that you can put in the same area of your site.
here is a site to get more technical idea on optimizing your CTR. Link if you can go through them, you can make it work, just make sure you check your statistics every week and experiment on ad placements. but as stated on the site, filtering can also enhance your site, i think it may be true to real estate blogs, but it would also be wise to experiment on this first rather than just be spontaneous.