He asked not to be. See #62.
http://blogs.wsj.com/washwire/2011/08/07/ryan-downplays-prospects-of-new-deficit-committee/
“Mr. Ryan said that House Speaker John Boehner (R., Ohio) hasn’t yet settled on which three House Republicans he will select for the 12-member panel. Asked if he would be on the panel, Mr. Ryan said that “I have not been told.”
He requested it. Notice he did not say “I am not going to serve.” He said he hasn’t been told yet whether or not he would be on it. The quote you cite was made AFTER the selection. I know what I saw and heard. Maybe he had a change of heart, I am not sure, but he asked to be on the committee publicly...