Xbox has never had as much of an RPG fanbase compared to Playstation. Playstation has had generations of great RPGS on its systems while Xbox has had... Lost Odyssey and Tales of Vesperia?
I definitely played RPGs on my 360 when I could, but as time went on, more and more publishers were going Sony-only.
I can only assume that for a more "niche" genre that maybe it's a way to save money, too.
It just seems strange given that the two systems (PS4 & Xbone) are so similar to program for and port games across.
Xbox 360 actually had quite a few RPG's if I remember correctly, several exclusives too. Last Remnant, Infinite Undiscovery and Star Ocean were all Xbox exclusives with Star Ocean getting a port over a year later to the PS3. Then you have others like Arcania Gothic 4, Blue Dragon, Dark, Dark Messiah of Might & Magic: Elements, Divinity II-Ego Draconis, Divinity II-The Dragon Knight Saga, Fable Anniversary, Fable II, Fable III, Happy Wars, Kingdom Under Fire-Circle of Doom, Magna Carta II, Ninety-Nine-Nights 1 & 2, Naruto-Rise of a Ninja, Operation Darkness, The Outfit, Risen, Spectral Force 3, Too Human, Two Worlds and Witcher II. All of those were Xbox exclusive.