Wa alaikum assalam,
The evident answer is we don't. The Quran
only expects that some sort of circumambulation is performed in the remembrance of God. It is not concerned in what direction it is performed.
In the
spirit of an ancient practice established at the time of Prophet Abraham:
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"Then let them complete the rites prescribed for them, perform their vows, and circumambulate the Ancient House (Arabic: bil'bait-il-ateeq)."And further as an Arab pagan practice which was allowed to continue in the name of the one true God:
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"Indeed! Safa and Marwa are among the Symbols of God. So whoever performs Hajj of the house or performs Umrah it is no blame on him that he circumambulates (Arabic: Tawaafa) both of them (Arabic: Bihima). And whoever performs his own impulse to good, then indeed God is He Who is All-Appreciative and All-Knowing"A practice must have been initiated by the early followers and this has continued traditionally. There is no religious prescription to perform tawaaf anti-clockwise in the Quran. However, there is also nothing wrong with following an established practice in situ.
Please see below a related article which may be of interest.
THE SEVEN CIRCUITS OF THE KA'ABAhttp://quransmessage.com/articles/seven%20circuits%20FM3.htmI hope that helps, God willing.
Regards,
Joseph.