I've had less than 0.5% of customers ask for IPv6 from my fibre ISP. It's not worth supporting as a result. The main reason is that any service that is not widely used will have gremlins that result in poor customer experience, and if it's always the same handful of customers hitting problems or finding quirks, there is a real risk of poor word of mouth incident reporting that can harm the business. At least if something goes wrong with IPv4, it's going to be noticed very quickly.
Some people will say monitoring is all that you need, but I do not agree. There are a million different little issues that can and do occur on physical networks in the real world, and there's no way monitoring will have a 99% chance of detecting all of them. When incidents like the partial Microsoft network outage that hit certain peering points occurred, I had to route around the damage by tweaking route filtering on the core routers to prefer a transit connection that worked over the lower cost peering point. It's that kind of oddball issue that active users catch and report which does not happen for barely used services like IPv6.
Some people will say monitoring is all that you need, but I do not agree. There are a million different little issues that can and do occur on physical networks in the real world, and there's no way monitoring will have a 99% chance of detecting all of them. When incidents like the partial Microsoft network outage that hit certain peering points occurred, I had to route around the damage by tweaking route filtering on the core routers to prefer a transit connection that worked over the lower cost peering point. It's that kind of oddball issue that active users catch and report which does not happen for barely used services like IPv6.