Although, we just might not have a 'need to know'.
What some critics attribute to a show of humility and poverty on the part of His Holiness, often accusing him of disrespect for the papacy, seems to me to be more appropriately attributed to genuine charity. For instance, ignoring protocol, going 'out of bounds' to greet the faithful or speak informally with visitors - I see that as a kindness arising from charity.
Therefore, if the Pope was a no show for a concert - if anything, I would suppose he felt obliged, in charity, to attend to a more pressing need.
That said, the Holy Father did send a message to the participants:
(Vatican Radio) Below is a Vatican Radio English translation of a message sent by Pope Francis to the participants of a Concert for the Year of Faith held on Saturday.
However, if you really feel you need to know more...I would like to say thank you to all those who made this concert possible and who organized it in the Year of Faith. In particular, I thank the soloists, the choir of the National Academy of Santa Cecilia, the National Symphony Orchestra, the Conductor Juraj Valčuha for the remarkable performance of this monumental Symphony, which not only makes us live a moment of pause and elevation of the soul, but provokes in us all feelings and emotions that drive us to reflection. Thank you so much. "