I think the main reason is that nobody in North America or Europe sees
any but hypothetical reasons to accept v6 mail any time soon. I do,
and I can assure you that 100% of the mail I get on v6 could equally
well have been sent on v4. If it were up to me, I would put all of my
v6 effort into web services, which loses a lot more with NAT or
proxies than mail does.
There's two basic problems with v6 whitelists: making them and using them.
For using them, we have a version of rbldnsd with basic v6 support,
but nobody has any real idea how well it'll work, because we don't
understand very well how DNSxL traffic caches. If you're a large
provider and can run rbldnsd or the equivalent on the same LAN and
have the sophistication to fetch or manage the DNSxLs it serves,
caching doesn't matter, but for everyone else it does. The limited
data I have is utterly unclear about practical v4 DNSBL cache
behavior. At the very low end, it doesn't cache and doesn't matter,
for some kinds of woodpeckering it does cache which does help, in
between, who knows.
For making them, the problem is how you come up with a registration
system that is simple enough that unsophisticated MTA operators can
use it, but hostile enough that bots can't script it, not unlike the
problem of webmail signups. If we could figure that out, we could at
least try some experiments.