Why are most smart switches on Zwave instead of Zigbee?

They are priced well but they have issues. I have actually used them a couple of times but you really have to check loads. If the load exceeds 3amps you will have a problem with them disappearing. Also you need a good quality router that can handle the switches and your existing traffic. I fixed a network one buy simply replacing one mesh network for another. When you buy the router check how many connections/devices that it allows; you wan to see 150 or so.