It's primarily due to router discovery and automatic address assignment combining with stacks built to prefer 6 if available making it trivial to hijack/reroute

It's primarily due to router discovery and automatic address assignment combining with stacks built to prefer 6 if available making it trivial to hijack/reroute/mitm your traffic. It's a significant concern on local networks that haven't intentionally deployed 6, you can see this behavior where things like laptops will inadvertently take over traffic on a network and push it through its 6 tunnel, generally evading network boundary controls.