Board Thread:Questions and Answers/@comment-25322628-20170117173017/@comment-1635567-20170120054122

According to Matt Price, during an AMA on Reddit with Calvin Wong (link to the source right here):

"We felt like the Margaret / CJ story was complete, and they were both on their own path already. We also felt like Mordecai had moved on in his life. He finally left the park to pursue art, and we thought it'd be cool if he met someone while doing that. But we love Margaret and C.J., and we're sure that they're happy without Mordecai! He was too wishy-washy with them!!"