If you sign up for IPv6 you don't just get one IP address but probably a minimum of 65,536! So there'd be not just be 65,536 ports to scan, but 65,5536 prorts to scan on each IP address! If you're wanting to scan your Internet-facing router there's no knowing which IP addresses the ISP might leave open to use to contact the router itself for management or updates. So in short, it's complicated.