IPv6 was obsolete by the mid-2000s, majorly due to the advent of roaming. It was designed on the rather fanciful assumption that its deployment would simply supersede IPv4, that every software/hardware vendor would cooperate, and we'd have a pure v6 network which would also replace the traditional L2/L3 layers.

Ofcourse legacy compatibility trumps all, along with the ubiquity of NATs and roaming and we're now just in the sunk-cost phase, being left saddled with a horribly bloated protocol (128-bit addresses was a marketing choice; not engineering) that solves no problems.