The reason being is that voice actor Harry Shearer is threatening to leave the show if the producers do not give him the freedom to work other gigs he says.
According to an article found on reddit and written by IGN.com, Harry Shearer does the voices for several of the characters on the hit FOX show. For example, some of the characters that he does include Mr. Burns, Principal Skinner, Otto the bus driver, Smithers, Ned Flanders, Timothy Lovejoy and Dr. Hibbert to name a few.
Daniel Amen (MyBrainFitLife) thinks that this is going to upset many people out there, because even if the show had to lose a character or two, this is way too many to go on without. Not only that, but it would be difficult for Matt Groening to replace the actor with someone else since these characters have had these voices for almost two decades pretty much. It will be tough to find anyone who can do nearly as good of a job as Shearer so hopefully the producers and him will be able to find a mutual agreement where they can both get what they want and still produce amazing episodes of the show.