Well, according to an interview, it was the show-runner's decision.
Which doesn't make a lot of sense; Carl figures a lot in the storylines after the Negan war, and they even established that he and Negan have some kind of rapport in the previous episodes, so why off him? I'm guessing the showrunners decided to take the blame in case the show's fans take it out on Chandler.
If they really want this show to move forward, they should just ignore the comics storylines and go their own way.