Because Ashli Babbitt was watching the members of the House, being directed and led out the other end (the west end) of the Speakers Lobby (a corridor that runs east-west, adjacent to the House Chamber).
Some Republican members of the House, typically come and go via the west end, but Democrat members of the House, come and go via the east end - a result of where the members tend to gather as groups - maybe tradition, habit, or routine.
Ashli Babbitt was just outside the doors of the the east end of the Speakers Lobby. THIS area, outside the doors, is one level of the east stairwell whereby people eventually have access to the east front steps outside the Capitol building.
Ashli Babbitt might have felt an urge to catch up with the departing House members whom she recognized, and not lose the opportunity to present her request that the 2020 election be investigated -- that was her mission.
In the image above, far left, there are 2 of the 3 Capitol Police officers that were at the east doors. The 3rd Capitol Police officer was off the right shoulder of the left-most officer (who is wearing a light-blue mask). The 3rd Capitol Police officer (not shown) was wearing a somewhat-green jacket. If you watch the following, very short video:
https://www.youtube.com/watch?v=ldqDgTlOfq4
you will see all three Capitol Police officers (mentioned) step aside from doors that they were guarding, and leave.