1. Because 9in risers are easier to walk up and one doesn't normally have to walk up an escalator, and 2. Because it means the staircase can be steeper, thus taking up less floorspace.

Escalator steps need to be long enough for those with larger feet to stand still comfortably on, in particular with their heels fully on when ascending.  The pitch needs to be such that the majority of users can walk up or down comfortably, but not too shallow or that adds unnecessary distance.  These factors combine to give the typical (visible) riser height observed.

In contrast, a fixed staircase is designed for all users (unless chair-assisted) to walk up or down comfortably, including most children; adults can take two steps at a time if they wish, and it is not so important for people with larger feet to be able to stand still comfortably.  In combination, a smaller typical step length and riser height result in this case.

Doesn't the geometry of an escalator mechanism dictate that the overall dimensions of each step "unit" be square? Therefore if the step is a foot long it will need to be a foot high.