Its pretty obvious that the biggest fanbase is on PlayStation but, Final Fantasy model have become quite unsustainable for it to be on Sony flagship console. The series obsession with chasing graphics since the original Playstation has backfired with FFXIII and FFXV. FFXV does have some amazing visuals but the game is broken, unfinished mess. Quite a spectacular failure from one of gaming beloved franchise.
FF being on the Switch allows it to break free from the chains of chasing graphics due to how low powered switch is and, focus more on story and gameplay. A 6-12 month exclusivity do wonders, get nintendo fans acquainted with the series again and later Playstation still get the benefit of upgraded visuals and good gameplay. Its a win-win situation.