There are already a number of threads about this in the forum, But the short answer is that people are used to Wi-Fi so they like Wi-Fi which creates a big market so right now those devices are often cheaper. But you will generally top out at 30 or 40 max devices per Wi-Fi router, and they use a lot more energy per device, Which means you won’t find battery operated sensors using Wi-Fi. However, WiFi is great for gigantic messages, like streaming audio or video.
Zwave and zigbee were designed for automation of very small messages sent infrequently. And they can handle hundreds of devices. Really good for sensors and light switches. They use very little energy to operate. Pretty much any professionally installed system that you buy for security or home automation is using one of those two protocols, or a proprietary protocol with similar characteristics.
If you have a smart meter or a cable TV set top box, you probably already have a zigbee network in your home, you just don’t know it. And it’s not likely to be one that would integrate with a home automation system. I’m just saying zigbee is used a lot, you just may not have heard of it before.
Here’s a good recent thread to start with about the different protocols (this is a clickable link)