With diesels the fuel cost difference (most marked at Euro 3 I think but still significant) of running a double decker over a single-decker, particularly a lighter weight option like an E200, was such that double-deckers were often only deployed where absolutely needed. With electrics I think the calculation is different but I suspect range issues may come in instead as single-deckers have less weight issues so can have larger batteries and run longer distances without recharge over a double-decker.
The X3 could really do with being double-deck, more so than some of the city services run with deckers being discussed as it can often be quite busy, and it would be quite easy to avoid going under that low bridge (just going back to the old route would do it) but they would have to get Stagecoach to also do it and they were the first operator to route under that low bridge.