They are operating exclusively Boeing 737 variants ( afaik 600+ planes) to save money by symplifying maintenance and pilot training. Adding another new completely different plane into the mix (one noone outside china knows how to maintain or fly) would add lots of costly complexity and overhead they can do without. They would have to fully switch only to single COMAC plane variant for all future purchases to make it make sense and imo they are nowhere near beaing ready fot that kind of commitment.