Some blame alcohol, others depression, but all we really know is that Pete Duel had the talent to be a major star. This is not only evident in Alias Smith and Jones, but in all of his work prior to that. He not only had good looks, but he had the charisma and talent to make up for any other short comings he may have had.
He showed promise in a supporting role in the movie, Hell is for Heroes, but it was television that really embraced him with major roles in the short lived shows, Gidget and Love on a Rooftop, as well as his dramatic guest starring roles in other popular shows of the time like The Bold Ones and Marcus Welby, M.D., just to name a few.
It's hard to predict what might have been, but anyone familiar with the work of Pete Duel knows that he could have and most likely would have been a big star, making him the pick of the day for Not Very Famous...but should be.