Good question and I guess there are many different answers, depending on who you are talking about. In many cases people don’t really analyze the situation or buy in for the belief that there is no need to look at IPv6 for different reasons, such as having enough IPv4 addresses, thinking that IPv6 will not come in the near future, having too many other IT projects on their plate or other reasons. What they don’t realize is that the Internet is a global creation and that everyone that profits from it should also contribute his part to it. And these days that would include to introduce IPv6, because IPv6 is about the continuation of the Internet.
But you’ll also find many other reasons why people don’t use it more often. Let me know
One of the big reasons is money. Globally deploying IPv6 will require a huge amount of money upgrading the core networking equipment so that it supports IPv6. No one wants to pay for these upgrades so until there is an actual reason to do it, the companies that own this equipment won’t make it happen.