It seems to me that there are disadvantages to widespread IPv6 adoption for some big companies. For example, storage providers like Dropbox or Google Drive would take a hit if I were able to send files via a direct connection to a friend's computer, which is possible with IPv6. Right now, the best way to send large files is to upload them somewhere and then send a link (dynamic DNS is also possible, but certainly not as easy to set up as simply being granted a globally routable IP address). Another example is for ISPs or anyone who sells static IP addresses (e.g. AWS); IPv6 means severely decreasing the scarsity of a resource that they own and profit from, which I'm sure they wouldn't like.

To generalize, I think IPv6 represents more freedom for end-users in the form of better p2p services, but I think it also represents a loss in profit for some large companies. These companies play a roll in the IPv6 rollout, so maybe it's not surprising that the rollout has been slower than expected.