Q. Kindly tell me why we chant “Hare Krishna Hare Krishna Krishna Krishna Hare Hare, Hare Ram Hare Ram Ram Ram Hare Hare” instead of “Om Namo Bhagavate Vasudevay” or “Om Krishnay Namaha”.

A. All bonafide scriptures instruct us to chant the holy names of God. In some traditions, the followers do chant the mantras mentioned by you viz. “Om Krishnay Namaha” etc.

We, the devotees of ISKCON are followers of Lord Shri Chaitanya Mahaprabhu, who specifically recommended the chanting of the Hare Krishna maha-mantra viz.
Hare Krishna Hare Krishna Krishna Krishna Hare Hare Hare Rama Hare Rama Rama Rama Hare Hare

So, we chant the Hare Krishna maha-mantra.