Jul 30 01:15 2018, Michiel van der Vlist wrote to Markus Reschke:
MvdV> What I find remarkable is that T-Mobile US is working towards an MvdV> IPv6 only mobile network by switching off IPv4 and accomodating MvdV> "legacy" with transition mechanisms. While here in my part of the MvdV> world they are still IPv4 only. Even their website is IPv4...
I assume they didn't get sufficient IPv4 address space from ARIN for the US market. So they were forced to go for IPv6.