One reason is that ZigBee got there first. It was the established player, people had worked around the wrinkles and it was generally pretty good. And you have a

One reason is that ZigBee got there first. It was the established player, people had worked around the wrinkles and it was generally pretty good. And you have a network effect of installed devices. The main advantage (AFAIK) of BLE mesh is that your radio can also easily do normal BLE which can be helpful e.g. to talk to phones. This hasn't (so far) been enough to make it worthwhile to switch from ZigBee.