Then why is it that people could take Sri Krishna to be a Supreme God over other Avatars?

I think those who does this are missing the very important lesson Maha Vishnu is trying to teach them. No matter how many avatars He took, He is the same. Those avatars are merely "Forms" which He needs to accomplish the tasks.

Same way, no matter how many times we are born, die and reborn again, we are the same regardless of our physical differences. We could have been born as animals, plants or humans of various races. But the Soul remains the same - unchanged and eternal. No one life or form in any one life is more important than another.

This is what differentiate Christians from Hindus. Christians stuck with Jesus. They cannot think outside the "Jesus Box". Hindus should not be stuck in the same "box" as well.