The other platforms don’t support Bluetooth headsets either, so this is hardly a dealbreaker. I’m guessing the reasoning is due to the audio lag that usually comes with Bluetooth. Bluetooth is fine for phone calls and music, but the lag in game audio would make games unplayable.
I am curious why the Switch has Bluetooth 4.2 though, when the joy-cons only support Bluetooth 3.0. If 4.2 isn’t being used for headphones, then what is it being used for?