The reason why VoLTE went IPV6 first has to do with the structure of the carrier networks in IPV4 space, where there is a huge amount of NAT'ing to conserve address space over on the Radio Access Network - you'll find a lot of 10'dot's and 172.16.0.0/12 ranges out there - made sense for a long time, until LTE came along with persistent connections, and then the IMS bearers, which cannot drop as the mobile moves thru the network.
Once the IMS core was set up, getting WiFi connectivity over L2TP was almost a no brainer - even over IPv4 for those that would rather not have IPv6 inside their LAN - one can tunnel the IPv6 thru the IPv4 connection.
The wireless operators, at least here in the US, have really led the way on IPv6 as a preferred transport over IPv4.