Escalators were made for the consistent flow of people who know how to step on and off one. Putting an escalator to the door of an airplane is asking for trouble. As we all know, the boarding process is known as "hurry-up-and-wait." Everyone rushes to stand in line at the door to the jetway/ramp, then after passing that door, they wait at the door of the airplane while everyone funnels through and those sitting in the back block the aisle by putting all their carry-ons in the front overhead bins. With an escalator, everybody will be piled up at the top next to the door with nowhere to go. If passengers are not piled up at the top, then you would have someone who stops at the top of the escalator to admire the view before boarding causing another pileup on the escalator.
Deplaning is another issue. Now you have mommy carrying her baby, a car seat, a stroller, a diaper bag, and two weeks of clothes for a three day trip trying to step on the escalator. Ramp boarding/deplaning is a favorite photo opportunity for travelers. I can see the family photo being taken right at the bottom of the escalator causing another logjam.
Another thought: If the escalator is going up and it's unattended, how do you evacuate the airplane through that door?