Because We Japanese walks so many times and we are work so hard, we do not have time to just stay and wait until escalator finally moves us to the next floor.
So, we made a rule. The rule is all the people who is lazy enough to not walk up or down the escalator needs to stay left side of the escalator and dedicate right side for the people who is in hurry and actually walk up or down.
If you just talk with your buddy right next to you and block right side of the escalator, it really pisses people off. Usually the people who walk even they are on the escalator is under high pressure in their environment. They tends to have hysteria. We all Japanese have hysteria in some level. All the people in the world do now a days actually.
Anyway, you will piss them off and they tend to think you guys are from country side where every thing moves very slow.
So, Do not stay on right side of the escalator!
There is one more thing you should know about. In Osaka, the city with people who has highest average walk speed, people walk left side of the escalator and save the right side of the escalator for lazy people.
I do not know the reason. It's just TIJ. (This is Japan)