Can't get the same metrics off the ads in an RSS feed. Companies, like people, are cheap so they don't want to spend money on advertising that doesn't work. RSS feed ads don't report back the same way browser advertising does.
Secondly, that just leads back to my first point. The ads (even targeted ads) are already on the final content.
I just can't view this as a failing of RSS. It wasn't intended to help publishers advertise, it was intended to help end users track the sites they wanted to track. I understand these are some of the reasons Google and other publishers disincentivized the usage of RSS but RSS works great for its intended purpose. I want more technology like RSS, not tech that helps publishers exploit us