Because it wasn’t implemented transparently at the OS. I can’t plug in to my ip6 only network and reach an ip4 address because end user OSes don’t implement clat transparently, even today in 2026, let alone 20 years ago.
I'd argue it has, but that's not the point. IPv4 is pretty much mandatory for most users of the internet. Yes, individual users probably neither know nor core whether they're on IPv4 or IPv6, but it's so difficult to run IPv6-only that you just wouldn't bother.
Trying to teach someone about how to use ULA vs. GUA only leads to headaches. In fact, I'd go so far as to say the reason why the internet became so ubiquitous was because of NAT -- static internal addressing is pretty much fundamental to an easy-to-use experience. Having multiple addresses on a host only makes things far more confusing, so people don't bother, because IPv4 justworks.
Because it wasn’t implemented transparently at the OS. I can’t plug in to my ip6 only network and reach an ip4 address because end user OSes don’t implement clat transparently, even today in 2026, let alone 20 years ago.