^ Yep, both reasons are exactly why this is.

This is the game Horii wants to make and the hardcore fanbase (both in Japan and overseas) wants to play but unfortunately SQEX does not have confidence in a PS4-only release in Japan, and for good reason. So they need an "insurance" version on a more popular system but because Unreal Engine 4 has no official support for any of them, they have no choice but to make a separate game. The 3DS is the logical choice because they already have the assets for the other games, which they are using to piece together DQXI 3DS. That ensures, however, that the PS4 version is dead on arrival in Japan, creating the ironic reality that Yuji Horii and the main development team are effectively creating a mainline Dragon Quest solely intended for Western release. It would be amazing if the mainstream Japanese market actually springs for DQXI PS4 but I won't be surprised if/when they don't.

Either way, both versions look fantastic and I'm so happy we're getting the first "real" Dragon Quest in more than 10 years.