I think that the downside to the van is that there'd only be one per rake, whereas with the other coaches, you could have four, six, eight or whatever per rake, yet the manufacturer would only need two different moulds to shift those eight coaches.
So with a van, one sale per rake per mould, and with the coaches, four sales per rake per mould.
Not knocking the vans in any way, i understand the importance of them, just looking at it from a practical point of view.
A van might end up costing €150 whereas the coaches might only cost €60. All guess-work, yes, but probably close enough to the mark.