I would guess that it is the cost of the battery vs the cost of the van. The trouble with vans is that they are cheap - a bog standard 2.0l transit only costs £18,500 and you won't get a lot of battery even if you trebled the price of the van.

A diesel double decker bus will cost between £250-300k depending on spec - an electric double decker will cost circa £500k with the battery being a significant part of the difference.