"The three other's did make a fortune and alas Adam failed in his subsequent career."
He was in movies and had a TV show on for several years, I don't think he was a failure and he did what he wanted to do. I remember one of the other 3 saying in Parade mag. in approx. 1965 he was giving up $500,000 a year with the advertising and salary.
"Adam added meat to the show and it worked for the years he was there."HondoDukeLane"
I agree with this statement, he was a fine actor. He had a manner that was more like a great Actor then a run of the mill guy. He added some class for me and I thought the show was less without him.
I found the following two paragraphs at: "http://en.wikipedia.org/wiki/Pernell_Roberts"
"Roberts, had had high hopes for what he could contribute to Bonanza, was disappointed with the direction of the show, the limitations imposed on his Bonanza character and on his acting range. In a newspaper interview he said, "I haven't grown at all since the series began...I have an impotent role. Wherever I turn there's the father image," (This Time Pernell Won't Need a Tuba, Washington Post, May 1, 1963, Lawrence Laurent).
Finally, after disagreements with writers and producers over the quality of the scripts, characterization, and Bonanza's refusal to allow him to perform elsewhere while on contract, Roberts "turned his back on Hollywood wisdom and well-meant advice," and left, largely to return to legitimate theater (Washington Post, January 25, 2010; New York Daily News January 26, 2010, Mike Douglas Show, 1965, 1966), Henry Darrow Archival Interview; USA Today, January 25, 2010)."
There is much more at wikipedia, but I believe this is the essence of why he left the show.