Cecil Terwilliger envied his brother for a lot of things. Bob had so many talents, and a fair bit of charisma, in addition to getting more of the scant attention from their mother.
That wasn't unexpected, as Bob's personality mirrored Judith's far more than Cecil's did. Bob and Judith were both intelligent, vain, vindictive, and temperamental, with a penchant for drama. Cecil took more after his father, being more placid and practical, and he disliked showing off his talents with excessive theatrics.
These traits may have wrecked Cecil's desire to perform, as he had little stage presence, and perhaps too much concern for his dignity. On the other hand, self-control had served Cecil fairly well in prison. For all that he had experienced in the purgatory of Springfield's prison system, Cecil had never been sedated, beaten by guards, or ended up in a straitjacket, and he still maintained his grasp on reality, unlike his brother. At least Cecil was better at something than Bob after all.