I suspect the big reason it hasn't happened yet is it would require ISPs to replace tens of thousands of dollars of hardware and it would increase support requests in the short term ("site XYZ is broken but it's fixed when I turn of IPv6").
I'm a software engineer with a smidge of basic networking experience so not completely clueless, but definitely inexperienced with DOCSIS and this sort of residential networking stuff.
Here in Germany, you no longer get IPv4 addresses from Unitymedia.