When the Borough of Luton decided to build an airport in the 1930s, they had the perfect spot for it. To the south east of the town was a large, flat plateau, raised above the Lea valley, which would be ideal for a runway. They opened an airport there in 1938, and for seventy-odd years London Luton Airport has operated out of this spot.
The problem with this location is: it's a large flat plateau raised above the Lea valley. Road and railway engineers choose the path of least resistance when building transport links and so they'd sent their connections through the valley. It meant that even though a busy railway line with fast connections to the capital passed less than a kilometre from the terminal building, getting it onto the airport estate would've meant huge amounts of tunnelling and bridges to overcome that massive height difference.
A station was built in the 1990s, Luton Airport Parkway, but that was still a mile away from the airport proper, and so shuttle buses had to be laid on. Until, finally, they built a dedicated people mover: the Luton DART.