The Controller, both in power consumption and size requires for those USB 3 Speed never quite fit into iPhone requirement. ( You can check out the size of USB 3 Speed lightning controller on that iPad, if my memory serves me correct it was 4x the size of similar controller for iPhone ) In 2020 this may no longer be the case, but then there is the BOM cost issue. Why pay more for this feature when you can do it wirelessly? WiFi 6, and in the future 802.11ay using mmWave provide 1Gbps to 10Gbps real world wireless transfer speed.