I'm no network engineer, but as I understand it, to support IPv6, companies need to replace their switches. I think it's fair to say that there are literally millions of switches that need replacing. We are talking billions of dollars in total investments. I really don't see how it's surprising that this will take a while. Billions of dollars don't hang on trees, companies need to earn the money before they can spend it.
At the same time, because IPv6 is used less frequently, it is more expensive. The price of electronics is determined by volume: the more you produce the cheaper it gets. This means IPv6 has a price disadvantage to IPv4, which is especially noticeable in the early years (of ~0.1% adoption). A device that is produced at only 0.1% the volume of the most popular devices will be considerably more expensive.
This is, in part, why we see an exponential adoption curve: the more people who buy IPv6 equipment the cheaper it gets, and the cheaper it gets the more people buy it, this chain reaction helps to cause the exponential adoption rate.
I'm not saying everyone will end up using IPv6, although I think it is likely, but I'm saying it should be no surprise that replacing billions of dollars worth of network equipment takes time.