He's become a huge fan-favorite, and has apparently given the writers lots of ideas and premises for episodes. His gullibility and weakness made for the perfect opposite to Cartman; lots of things could happen with these two characters together.
Although if you ask me, the extreme focus on Butters might be slowly fading. After "Sexual Healing", "Scrotie McBoogerballs" and "Poor and Stupid", he sat the entire fourteenth season out.
Maybe he'll come back, maybe it's his time to go. All I know is the show is always changing, and it ain't big enough for every character to fit.