Hehehehe....Gee, I wish we had those "crap railways" in the USA, Englandair. I suspect you're right though, the cities are just too close together to make an airline shuttle popular at a price people would be willing to pay. If the railways have problems, people can still drive or take a coach in not much more time than it would take to get to the airport from the city, check-in, board, fly, claim baggage, and get to city from the airport.
In the US, Boston and New York are about 190 miles apart. Yet, until very recently when new trains were introduced, the train journey took 4-5 hours. Driving takes nearly as long, and often longer if there is traffic. Since BOS and LGA are relatively near their cities, and served by public transport (trains to BOS, train and bus to LGA) or a not-too-expensive taxi ride, air shuttles have been successful and justify (perhaps) their high prices. Right now, I can fly from BOS to LHR *for less money* (3300 miles, on a restricted ticket) than I can fly to LGA (<200 miles) on an unrestricted shuttle ticket.