Why Krishna Has to Come When His Other Avatars Will Do the Work

Krsna states it in this very well known verse of the Bhagavad Gita:
"In order to deliver the pious and annihilate the miscreants, as well as to re-establish the principles of religion, I advent Myself millennium after millennium." (Bg. 4.8)
Krsna always has a desire to protect His unalloyed bhaktas, annihilate the wicked, and to re-establish dharma. He therefore descends in every yuga. However, this time when the Lord came as the incarnation for the Kali-yuga[to spread the glories of chanting Hare Krsna], He incorporated another special motive. That motive was to distribute the process of devotional service on the platform of transcendental spontaneous love. "My plenary portions can establish the principles of religion for each age. No one but Me, however, can bestow the kind of loving service performed by the residents of Vraja".

http://www.krishna.com/node/118