IPv6 spec was 1996.

Four years is not really a very long time in the greater scheme of things, the pace of modern web development just makes for an impatient populous with short attention spans.

I don't think that you can compare a backwards-incompatible change in a low layer of the stack that runs the internet with a feature of CSS parsers that was standardised 4 years ago. Some things are expected to take more time than others.

If v6 were widely deployed in the early 2000s, then the Windows botnet phase would have been so much worse, many other issues aside.

There are browsers out there, that has not been updated in several years, like my Nokia N9 (Microsft bought the company and killed it)

Traditionally perhaps, but as we've moved into the age of evergreen browsers things have picked up substantially. It's now feasible to start using new APIs weeks after they roll out, sometimes with a polyfill to fall back to.