Etiquette and consideration for other passengers aside, this is based on the machine’s make. Popular escalator and elevator company Otis, has issued a statement to The New York Times, saying that its longtime position has been for passengers to stand still as a matter of safety. Walking on the escalator increases the machine’s probability for damage. Escalators weren't made for walking.
Walkers who want some exercise should take the stairs instead. Many countries have found that a larger number of escalator users take up the standing lane. You can see the imbalance this creates when reserving half the space for a smaller percentage of the population.
Those who feel the need to reach the next level quicker only save a few seconds by walking as compared to standing. If everyone stood, it would also lessen the average time on the escalator, including time spent in the queue to the standing