Why would they divert resources to a DOCSIS upgrade when they have an ambition of doing FTTP overlay of their entire network by 2028?
Easy question, short term time profit, relative ease of upgrade. They will keep more customers previously on 1gbps packages, by offering the highest widely available speed, compared to almost all FTTP connections. Many customers on Virgin's network don't necessarily care if the network is FTTP or DOCSIS based.
Virgin is not planning to expand their rollout much to many towns and rural areas (at least, going by what they've said so far), so it makes sense to upgrade the existing infrastructure.
I'm not sure if they will upgrade to DOCSIS 4.0 though, it might be preferable to lay FTTP cable in most areas instead, especially if there is demand for speeds such as 8-10gbps.
Prices may have to come down though, particularly as project gigabit and Openreach's FTTP rollout's near completion.
MPs have recently complained that the Government is relying too heavily on suppliers like Openreach and Virgin to complete Project Gigabit, so Virgin will still have little /no competition in many parts of the country.
For millions of people in FTTC only areas getting around 50mbps, a 2gbps connection would offer approx. 40 times faster download speeds, so an easy sell to those that could afford it. With similar latency too, especially with further upgrades.