I'm going to turn this on it's head. Why would someone deploy IPv6 today when they have working IPv4, no CGNAT, and enough addressing to last them for a while? There are 3 reasons I can see:

1. More IPv4 is expensive. Let's say the price is currently $50 per IPv4 address, but it's not an expendable resource... You could depreciate it over 4 years and say it's $1/month, but realistically those addresses will probably increase in value for at least the short-medium term, so it's really just minor. Especially considering the cost of the CPE etc, which are generally disposable after a few years.

2. CGNAT is expensive. Well, it can be, but that's mostly because it's stateful. If MAP-T takes off as well as it seems to be doing, then you can pretty much rely on stateless transformation, and that will be cheaper and cheaper as time goes on.

3. Customer demand. The only customers who are demanding it tend to be the whiny folk who also complain about Android not supporting DHCPv6, or that they can't pay their bills with some kind of cryptocurrency. They will always be complaining about something.

IPv6 is technologically superior to IPv4, there's no doubt about that. It is vastly inferior when it comes to understanding what is going on by your average sysadmin, network engineer, IT helpdesk person -- it is just far too complicated. Even the wording is confusing, e.g. router/neighbor "solicitation/advertisement" instead of "request/reply".

I am not just referencing the longer addresses, the "multiple addresses on an interface" thing takes a long time to get used to, having ULAs for internal addressing and then using GUAs (which may change with no notice) for internet traffic, it just confuses people. Expecting those users who run a server at home (a NAS or similar) to now rely on SLAAC rather than static addressing that they might have done before, or even worse configuring static addressing based on the prefix today and 6-12 months down the line suddenly finding they can't access the NAS any longer in their bookmarks, and no idea what to do to talk to it. They're not going to understand why people use ULAs, or which prefix to use there.

Not to mention that it would be a technical support nightmare to only offer IPv6-only services to customers, logging into their router or explaining why their few pieces of software not working may be because of the lack of IPv4 -- we're going to be stuck with IPv4 in the residential access and enterprise space for a long time, so there's very little incentive to put all the effort into IPv6 except for ideological reasons.

IPv6 is a case study in how all too often human factors are not considered in the design of engineering projects. IPv4 works, and is relatively inexpensive. Until it isn't, I absolutely understand why an ISP would not consider IPv6 a priority at all. IPv6 cuts your IPv4 costs, but has costs of its own.

... and I don't really know how to fix that.

M