I don't know exact reason giving CSFB to high priority over the VoLTE. But in my personal view, for voice service, lte network need to setup a bearer with QCI = 1, which ask to reserve more resources. In LTE , objective is best use of radio resources. Since voice packets come with some interval(few milliseconds) i.e. no continuous bit stream, no re-transmission, reason being operator would like to prefer CSFB rather than creating a dedicated bearer to serve voice.
There may be another set of reasons to prefer CSFB over the VoLTE.