Dave, quite simply because they have found thatthe present colours are acceptable to the majority of buyers.
Secondly, colour choices are fads, and peoples likes change very frequently, if a maker was to produce different coloured vans, they may well be left with many of one colour which they cannot sell.
Unlike cars, vans do not tend to change spec very often, and then only minor thngs, and the van market is not as rapid as the car market.
Itis also down to cost of parts. It is much cheaper to produce one colour than diferent colours, hence sticking to the lighter colours.
From a road safety point of view, White is seen as one of the safest colours, easily seen in all conditions, unlike some colours where the vehicle becomes almost invisible incertain light.
Lastly, it is what is acceptable to many councils who grant site licences, and what they see as acceptable "uniformity" and minimal visual impact