This has been asked many times. The answer is simple. Virgin is not Freeview, just as Freesat is not Freeview. They are all separate commercial TV platforms. While these platforms offer a service that is free to customers, they are not free to the channels owners, who have to pay out separately to each TV platform they decide to join. Hence it is a commercial decision by the channel owner.
Also Virgin & Sky are basically pay platforms, so some of these channels can opt to join one of the subscription packs in order to get some money back for joining the TV platform. Hence some may be on Freeview, but you have to pay for them elsewhere.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV6), Talk Anytime Phone, VIVID 350, Virgin PAYG Mobile