Thomas was a young and rising star, and he felt it was time to explore other avenues in his career. He was also interested in pursuing a college education.
The show's creators understood his decision and wrote out his character in a way that allowed for a satisfying conclusion to his arc. Randy's departure was explained as him heading off to college, which was a fitting ending for the character's growth throughout the series.
It's worth noting that Thomas's decision to leave was not a result of any behind-the-scenes drama or conflict. He simply felt it was the right time to move on.