In my opinion it's due to the fact that ads became very intrusive lately - video, audio, flash, popups, fullscreen overlays, endless scripts, you name it.
I don't know the solution to fix this mess, but I will gladly help pop the current bubble so something more useful can replace it.
There is also common banner dimensions that get blocked, but it is easy to defeat by adding a pixel somewhere that fits into site design.
Problem with this approach is that you can't just slap some random ad network script there and be done with it, no targeted ads, tracking etc - basically everything people don't like and a reason to use ad blockers.
EDIT: rephrase