Because that was only ever introduced on the tube.

If you are in any way observant, you will notice that the escalators and travelators on the tube are wider than the ones at airports and most department stores. With the amount of baggage the average air traveller carries, the practice of shoving people out of the way and overtaking on escalators is positively dangerous and, certainly at Heathrow and Gatwick, accounts for a high percentage of accidents. That's why barriers have had to be erected at the top of the escalators to try to slow the "numbnuts" down.

Patience with one's fellow travellers is the orfer of the day. It is not their fault if you have allowed insufficient time to reach the airport or your departure gate!