Telenet in Belgium has a complete IPv6 dualstack rollout on DOCSIS, so it's definitely possible. They've had it for quite some time now, and they're the reason why Belgium is near 50% Google traffic over IPv6.
It really just takes some planning and some guts to make the switch. If done right everyone benefits.
(Have to add that Telenet has the advantage that they have full control of every customer's modem, which usually includes the home network router / gateway, which does simplify a good rollout.)
Comcast uses DOCSIS and provides native IPv6. I think reasons are not technical - ISP needs to invest money to deploy IPv6 and ROI is not always > 1 for now.
What about the shared medium nature of DOCSIS? Do they share some infrastructure with the other cable ISPs or are these usually separate?
Usually this provider is very progressive [1][2][3] and because of these positive impressions I did not suspect them to hold back the cable IPv6 rollout due to financial reasons. According to [4] they even provide OpenWRT instructions for their native IPv6 and only their cable users are left without native IPv6. :(
[1] https://mirror.netcologne.de/
[2] https://www.sixxs.net/pops/netcologne/
Now, SixXS is sunsetting and the same provider still has no native IPv6 for their DOCSIS customers. Why is DOCSIS a special case here? Is it harder to roll out IPv6 for us because of some technical reasons?