The reason is that time applies a natural constraint on development. 
In other words, it’s just not been done yet and we’re working on it.
In principle, the new audio engine in UE4 naturally supports ambisonics decode the same way it would with other frameworks. I.e. multichannel audio in ambisonics format (or real-time encoding) would get sent to a submix mix and an ambi decode “effect” would decode the ambisonics format into the desired output format after applying the appropriate transformation according to the listener transform. However, the new audio engine pipeline is brand new and 3rd party developers are currently learning how it works. Hopefully, we’ll get native ambisonics file playback support soon as well as via 3rd party spatialization plugins.