actors. Unless they have other stuff planned for him to do, what is the point of having Patrick stay?
And if he stayed they would need to pair him with someone else and have him be in another storyline that didn't involve Robin at all and her fans seem adamant that shouldn't happen. Characters get written off all the time because they're not needed when another actor is either fired or decides to leave the show.
And IMO if TPTB were interested in the character of Patrick at all then they wouldn't have written off Matt. They could have written something decent for Patrick but from what I've seen onscreen, it appears that RC/FV have no interest in his character.