I'm wondering if it is something to do with the fact VM don't wholly own the XGS-PON network? Perhaps this is why they can't use modem mode, maybe this would be difficult for VM to manage/monitor the Hub 5X hardware?
Of course if VM had allowed access and terminated the fibre via a suitable 10Gb Ethernet/SPF+ ONT, people could have plugged in their own Routers aka Openreach style and this issue wouldn't exist. However as the vast majority of users just want "plug and play", I'm sure it's a Business decision to get the cheapest hardware out without any nod to the 1% of geeks and nerds hardcore networking enthusiasts (myself included btw) who want modem mode.