I don't use my van for any extreme off roading, I can use tractors for that. In the Alps they can justify using proper winter tyres, I can't be arsed changing tyres twice a year, but a T6 4Motion with winter tyres all round will still go further in the snow than a 2WD with winter tyres.
If I was buying new I'd spec the diff lock, but I found it difficult to find the used van I wanted with a diff lock. I phoned several companies about vans and asked if a difflock was fitted and most replied, "Err, dunno", only the switched on ones realise it's a selling point and they make that clear in the advert! As it has turned out I've never come close to getting stuck or not being able to get where I want to go.
Lots of people buy a 4Motion for its on road advantages, so a diff lock isn't really relevant for them. I doubt many people use a T6 and go more off road than a wet camp site, of course there will be some exceptions, but not big numbers.