Due to the scope of the project, they had to choose from one or the other
Final Fantasy Crystal Chronicles is making a comeback on PS4, Switch, and iOS and Android devices on August 27, but it will be doing so without a feature of the original. According to the official Japanese Twitter account, Final Fantasy Crystal Chronicles Remastered Edition won't support offline cooperative play.
Due to the scope of the project, the team had to choose between offline or online co-op. They went with the latter, which includes cross-play between platforms, so offline co-op had to go. The first entry on GameCube used Game Boy Advance systems and link cables for local co-op, allowing more info to be displayed on the separate screens, so it makes sense that they couldn't pull this off on all platforms. Still, some returning players aren't going to be happy about the decision.
Here's the new features trailer from June: