Comcast (an US cable ISP) starting deploying IPv6 because they have so many CPE devices that they are out of IPv4 addresses to do management on them. Story from 2010:
* https://arstechnica.com/tech-policy/2010/01/comcast-running-...
It's also why Apple mandated at some points that iOS apps had to work on IPv6-only networks: it was a requirement from mobile phone companies, as IPv4 addresses are getting scarce, and mobile telcos don't want to shell out cash, so they use IPv6 on phones and do NAT64.
- Both corporations probably started allocating from the top
- Both corporations probably allocated huge subnets.
I know IPv6 network engineers despise RFC 4193 - but, it offers extraordinary flexibility around addressing - particularly if you have large networks that you want full control over. At my last gig, we deployed over 25 million nodes in RFC 4193 space, and it worked like a charm.