They can. but that will increase the fuel consumption in turn, not to mention making the product line overly complicated. Double deckers deployed on interurban services remain a relatively small niche anyway, comparing to citybus operations, and with much of that market occupied by Scania and Volvo already, there really isn't much left for ADL to justify the investment of bringing a bigger engine E400 into the market.
Unless, of course, Stagecoach decides to play ball.
