Tradition is everything

A hundred years ago, the London Underground decided to adopt an American invention known as the "escalator" for the Earl's Court Station.

Must be true because they have a depiction
 in the London Transport Museum)

Unlike modern comb type, these escalators had a shunt mechanism ending with a diagonal (see pic): it finished sooner for the right foot than for the left. People were asked to stand to the right to allow others to pass
On the first day, a one-legged man was said to have been employed to ride it and demonstrate its safety.
Unlike other places where the escalator rules follows the side of the road people drive, the London convention of walking on the left on elevators was set so Londoners, still stand to the right.
You can read more about the introduction of various forms of moving walkways to London here.
If you are wondering the word ‘escalator’ was coined by one of the device's American inventors, Charles Seeberger, in 1895. He combined ‘elevator’, already a known term, and ‘scala’ the latin for steps – hence ‘rising steps’.