Why is pradakshina done only in a clockwise manner?

The reason is not, as a person said, to avoid a traffic jam! As we do pradakshina, the Lord is
always on our right. In India the right side symbolizes auspiciousness. So as we
circumambulate the sanctum sanctorum we remind ourselves to lead an auspicious life of
righteousness, with the Lord who is the indispensable source of help and strength, as our
guide - the "right hand".
Indian scriptures enjoin - matrudevo bhava, pitrudevo bhava, acharyadevo bhava. May
you consider your parents and teachers as you would the Lord. With this in mind we also do
pradakshina around our parents and divine personages.
After the completion of traditional worship (pooja), we customarily do pradakshina around
ourselves. In this way we recognize and remember the supreme divinity within us, which
alone is idolized in the form of the Lord that we worship outside.