How do we know that he didn't and they refused to release him from his contract? If I was Berman, Piller, or Taylor, I'd be really reluctant to let my second lead go right after losing my lead actress.
Bailing on a contract can get you blackballed. They don't call it a "breach" for nothing.
Folks getting released from their contracts at their request happens all the time, though. Look at Denise Crosby and Wil Wheaton.
Heck, when George Clooney chose to stay for the full duration of his ER contract after becoming a movie star, that was viewed as an unusual move.