The reason is that the Olympic Delivery Authority (ODA), which is responsible for "planning and delivering transport infrastructure and operations to support the Games," required that no hydrogen be stored at the buses' refueling site in Lea Interchange in Stratford between July and mid-September 2012 "in the interests of security," reports London SE1.
Since going into service, the buses have refueled over 1,000 times, writes Bus and Coach. Hydrogen supplier Air Products' Diana Raine said the buses have been "a great success to date," even though Bus and Coach also says the vehicles have been out of service almost as much as they have been in use because of maintenance issues and difficulty in getting spare parts. In any case, we're pretty sure that diesel fuel can be dangerous, too. It's not like the Olympic security rules make a whole lot of sense. WiFi hotspots, for example? Not allowed.