Because in the materialistic concept of life we can not see Krishna He most mercifully agrees to appear in the form of the Deity. The Deity IS Krishna, but He manifests Himself in a form that appears to be made from material elements that we can see. So in the temples you will find the form of Krishna appearing to be made from marble or metal but actually it is Krishna standing on the alter and Krishna will reciprocate with us in accordance to our surrender to Him.
An amazing world-wide tour of ISKCON temples enabling you to take darshan of Radha-Krishna all over the world. This video is available in super-high-quality when you purchase the Darshan Deity Book. Produced by Krishna Vision.