Because it is the only tune which should be played today; and preferably, repeatedly (although ideally it would be sung enthusiastically by a congregation that can't carry a tune in a bucket and doesn't care. This is not a time for choral perfection.)
The Pope is seeking to reconcile with the Lutherans, after 500 years. He will not, however, take communion with them. That schism still holds, though probably it holds more among clergy than among congregants, who don't really understand the difference between transubstantiation, consubstantiation, receptionalism, and memorialism.
The only time those differences have been truly set apart was in Prussia in the 19th century, when the Lutheran and Reformed branches of Protestantism were forced to reconcile. Their solution was a particularly elegant one which neither split the difference nor forced a false resolution. That resolution continues in a simple phrase now to be found only in abandoned copies of "The Hymnal" of the Evangelical and Reformed Church, in the prayer offered after the communion:
"May the Holy Communion strengthen and preserve you unto everlasting life. Be it unto you according to your faith." Amen. "Depart in peace."
"Be it unto you according to your faith."
Is the cup the true blood of Christ, the bread the true body, changed by a miracle from ordinary elements? Are Christ's body and blood present "in, with, and under" the bread and wine? Or is it that Christ is spiritually present, but not literally? Be it unto you according to your faith.
Even this is too broad a line, or too little a boundary, for some. I find it a great comfort, and a great sorrow that the words themselves are all but lost now, and soon will be. Thus is what we call "progress" too often lost and never found again.
Or maybe, as the example of Pope Benedict XVI and Pope St. John Paul II show, what really matters is your faith, your heart; what is most important is that it be unto you, according to your faith. And maybe there is a chance for progress, for improvement, after all.