So why did the kid who plays Dicky leave before the show is over?

i heard he got fired.

I heard he wasn't getting along with the other cast members and so they fired him.

Why Mace Coronel Left The Cast Of Nicky, Ricky, Dicky & Dawn

https://screenrant.com/nicky-ricky-dicky-dawn-show-cast-mace-exit-explained/

According to Deadline, a spokesperson for Nickelodeon stated the actor and network had “mutually parted ways” and that Mace Coronel leaving the Nicky, Ricky, Dicky & Dawn cast was “a mutual decision.” Coronel’s Instagram post suggested the same, stating it was a “mutual agreement” and that he was looking to take on “serious and more diverse projects” while assuring Nicky, Ricky, Dicky & Dawn fans the show would continue without him. However, Deadline also reported there were rumors of tension and personality conflicts between the show’s young cast members although neither Nickelodeon, Coronel nor any of his co-stars have ever addressed those rumors.