1. I'm a small-time self hoster. I need/want to control access to geographic locations and using IPv4 makes that pretty easy. Last time I checked, IPv6 was just so wrong that it's no good to use at all, and most IPv6 addresses were "unknown" in origin.
2. I'm used to the pseudo security that a NAT gives. I hear (ad nausea) about how NAT gives you no security. The simple truth is that obscurity does give yet another layer of protection, especially for machines that you're busily configuring to become secure. Of course, a real sysadmin here will be able to scoff and laugh, but for this homebrew old timer it's true.
But also, there's the obfuscation of your IP address when you're behind a NAT. This is pretty important in these ad tracking days. AFAIK (which is almost zero), doesn't IPv6 give adware companies a very good fingerprint on you?
3. All those ICMPv6 messages sniffing (and snooping?) really don't fill me with joy joy happiness. The only recourse is to read some dead boring RFC that my poor overloaded brain doesn't really want to have anything to do with. With IPv4, if you don't want PING, you turn it off, with IPv6.... it's subtle.
4. Firewalls require two sets of independent entries for the same service.
So, IPv4 is an address space that's understood. IPv6 really does feel like a Godzillian monstrosity and a chore to type in: double colons between every number and it's in Hex? At least with IPv4 you can type the numbers in pretty rapidly on a numpad.
So, I know this answer will be unpopular, especially to professionals, but for everyone I've encountered that's turned it off, the above list is pretty accurate.
Every few years I search around for a true "IPv4 to IPv6 for noobs" and every time I only find "It's the same... with these gajillion subtle differences". So, yeah, perception is definitely an issue, but scoffing at NAT doesn't help uptake at all. I really do try to be a good netizen, but it's hard. (that said, I do have a mail server set up to use ipv6 and all is good... except for the lists of unknown sources who hammer away at its security all day, every day)