Only reason which has been given so far is poor performance of this program for users, but other reasons like fraud referrals etc might have affected this decision.
I did try this service for few months and had few sign-up's too. Only one of the sign-up converted and got me $150.
We hope google comes up with a better referral program which can make adsense publishers more happier and richer
Please note that links you've created that lead to a customizable Onsite Advertiser Sign-up page will soon re-direct to the main AdWords sign-up page, but you won't qualify for any referral payments from Adsense. If you own a big website, and display only adsense Ads it's still worth putting a link up so that interesting customers can sign up for adsense and can have placement targeting on your website.
Inside AdSense: Farewell Onsite Advertiser Sign-up
