It just builds on actual HTTP, i.e. you have to actually think about the transport layer in a way that’s out of fashion now.
And some people don’t like XML.
It’s certainly true that CalDAV specifically is complicated, but… multi user calendaring is intrinsically very complicated.
Other systems are not less complicated, they just tend to be more vendor specific.
It just builds on actual HTTP, i.e. you have to actually think about the transport layer in a way that’s out of fashion now.
And some people don’t like XML.
It’s certainly true that CalDAV specifically is complicated, but… multi user calendaring is intrinsically very complicated.
Other systems are not less complicated, they just tend to be more vendor specific.