Maybe Apple company are researching to make VR headset in a new level when they release one. There are a lot of VR headsets right now and I think they know that if they release just the same kind of device then it will not really impact too much. I also agree that VR is not really mainstream right now. Maybe they are also want to release one if majority of the users are interested on buying one. Another factor is the apps. I think there are only few Apps where you can use VR headset at the moment. Maybe if there are a lot of games or Apps in the market then they will release one.