IPv4 addresses are easily enumerable, and this is a security problem; people looking for instances of vulnerable software will routinely try every IPv4 address

IPv4 addresses are easily enumerable, and this is a security problem; people looking for instances of vulnerable software will routinely try every IPv4 address (or search Shodan, a search engine for what-services-are-running-on-each-IP-address). With IPv6, you can't really find random devices that aren't hooked up to DNS or otherwise trying to be found; guessing IPv6 addresses is more akin to guessing passwords.