why should comparison site take the risk for your bad usability, design, etc.?
they send the traffic and you are supposed to convert it.
if you did your job and your site converts like crazy - you certainly pay less for CPC than you'd be paying for CPA.
on the other hand, if your site is crap and people simply do not want to buy from it - you certainly pay more for CPC than you'd be paying for CPA.
but hey, who'd like to send you free traffic if it helps nobody? just think of targetted adwords traffic.
I am sure, soon some comparison sites will come to best merchants and will ask them to work on CPA basis.