In 2010 Pope Benedict explained why those who received Communion from him personally must kneel and receive on the tongue:
The idea behind my current practice of having people kneel to receive Communion on the tongue was to send a signal and to underscore the Real Presence with an exclamation point. One very important reason is that there is a great danger of superficiality precisely in the kinds of mass events we hold at St. Peter’s, both in the Basilica and in the Square. I have heard of people who, after receiving Communion, stick the Host in their wallet to take home as a kind of souvenir. In this context, where people think that everyone is just automatically supposed to receive Communion — everyone else is going up, so I will, too — I wanted to send a clear signal. I wanted it to be clear: Something quite special is going on here! He is here, the One before whom we fall on our knees! Pay attention! This is not just some social ritual in which we can take part if we want to.I once read a quote from a non-Catholic who said if he believed what Catholics say they believe about the Real Presence, he would crawl up to receive Communion on his hands and knees. Does my disposition show this kind of belief and respect for the risen Lord who comes to me in Holy Communion?
Oh, how I wish we would return to using the altar rail and kneel for Communion. It not only shows respect for Jesus in the Eucharist, but is the great equalizer where college presidents kneel side by side with their students, where company presidents and CEOs kneel next to their employees, where presidents kneel beside secretaries and clerks, etc. There is no favored place at the table when one approaches the Communion rail. How I wish its use would return. I think our pope emeritus would approve as well.