1) IPv6 adds a ton of new features, and thus, complexities. It’s difficult for average users (even techies) to get into it.
2) Very few ISPs offer IPv6 end-points. You still need a real route to make use of IPv6, and if you’re a regular user, you need to tunnel IPv6 through IPv4 by getting an account with an IPv6 Broker. It’s often free, but the main problem is that not only does IPv6 have some additional overhead, but you have the overhead of being encapsulated in IPv4.
3) One of the MAJOR reasons IPv6 was born, was the threat of running out of IPv4 address space! However, we’ve mostly solved that problem by introducing NAT, dynamic IPs, and requiring legitimate reasons for people to have blocks of IPs.

IPv6 *IS* coming, just slowly. If I had to gamble, I’d say you won’t see it ubiquitous until the IPv4 address space is near completely used.

-Kevin