Because that's what a spinoff is, when a character from a show leaves to appear in his own show.
If you mean why did he within the Family Guy world, he divorced Loretta and wanted a new start.
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Realistically because the spin-off, The Cleveland Show was created. Within the plot line of the show, Cleveland lost the house to Loretta in the divorce so he decided to leave Quahog and move to California to be a Baseball scout.
Now that The Cleveland Show is cancelled, the Browns will be returning to Quahog in Season 12 of Family Guy.
Realistically because that's what the writers wanted to happen but within the Family Guy world...
He moved away to start a new life in California after divorcing Loretta.
Technically, Cleveland leaves Quahog in episode one of The Cleveland Show, but the last main appearance of Cleveland in Family Guy was in Road to the Multiverse where he is a Disney-style skunk in the Disney universe. He is mentioned in the next episode, Family Goy, as having left.
Cleveland isn't on 'Family Guy' anymore however his wife's name was "Loretta", his current wife's name on 'The Cleveland Show' is "Donna".
Cleveland Brown
Seth McFarlane and the people who made Family Guy
Cleveland Brown's wife.They divorced and then she died.