> prefers it over IPv4

That's the problem. If the OS starts using IPv6 preferentially but the software on top can't handle it, then you get a crash.

E.g.: if you turn on IPv6 for DNS and it starts returning AAAA records instead of A records, then a lot of applications fall flat on their face.

Usually the type written in C and insisting on maintaining compatibility with whatever Berkley did in the 1970s.