Why were the disciples unable to understand Jesus washing their feet?
They expected Him to come as a royal conqueror, like a Roman general on a white steed, not as a lowly servant. So they could not understand how the Creator and coming King of Israel and the whole planet could kneel and wash their feet.
Les explains that this is what threw such a curve at the Jews. They were looking for someone to come in on a white steed, like a Roman general or a Roman emperor. But here He comes as a servant. Christ was not presenting Himself as the royal king as yet; He was coming more as a servant, lowly, riding upon the foal of an ass. He epitomized that servant role when He washed the disciples' feet. They couldn't understand how the very Creator of the universe, the one who had come to be king not only of Israel but of the whole planet, could get down on His knees and wash the disciples' feet. This contrasts with Adam, who had dominion and fell, while Christ comes as the lowly servant, not as Adam with total dominion.