Because of 1 out of every 4000 users are "broken" based on industry measurements. This has delayed the web sites from publishing to IPv6.
When you go to your favorite web site in a browser, and it hangs for several seconds or even minutes. If the publisher publishes to IPv6, and your computer has a *broken* IPv6 route (as compared to having no route), then your computer will attempt to use that broken route first, before falling back to the IPv4 Internet.
Newer operating systems have IPv6 enabled by default. Older ones, may have IPv6 enabled, as a result of user action (or specific applications). Enabling IPv6 does not automatically make you vulnerable.
The problem lies when your system is convinced to try and use IPv6, when it is not working adequately. This may be because you're using a public service for your IPv6 gateway. Or, someone on your network, was advertising (to the network) IPv6 through Internet connection sharing; and they left. Last, your router may have IPv6 automatically configured to use public unmanaged gateways; and is telling your computer to use it.
The simplest, yet near-sighted, solution, would be just to disable IPv6. However, over the next couple of years, this will start to cause you more troubles than not. It would be best to instead find the root of your actual problem, and fix that instead.
The Broken User FAQ has several suggestions for broken users to follow, to remediate their situation.
This is an open-source project. I can certainly use your help in a variety of ways; please see the source page.