Why did St. John Paul II add the Luminous Mysteries to the Rosary?

The direct answer would be from the meaning of Totus Tuus.

Totus Tuus ego sum [Latin for "I am completely Yours" – Totus Tuus: 'Completely Yours'

St. John Paul II thoughts, words and actions are all coming from Our Lady's inspiration.

St.John Paul II embracing the fullness of Totus Tuus, meaning he sits and learn at the School of Mary, and perhaps, he reaches the height of contemplating the life of Christ in its fullness, seeing the light on the mystery that unfolds in His public ministry. This is the source, of St.John Paul II inspiration in adding the Luminous Mystery.He learned it, at the School of Mary.

With the Rosary, the Christian people sits at the school of Mary and is led to contemplate the beauty on the face of Christ and to experience the depths of his love. Through the Rosary the faithful receive abundant grace, as though from the very hands of the Mother of the Redeemer.

Against the background of the words Ave Maria the principal events of the life of Jesus Christ pass before the eyes of the soul. They take shape in the complete series of the joyful, sorrowful and glorious mysteries, and they put us in living communion with Jesus through – we might say – the heart of his Mother. http://www.vatican.va/content/john-paul-ii/en/apost_letters/2002/documents/hf_jp-ii_apl_20021016_rosarium-virginis-mariae.html

It was an inspiration coming from Our Lady of Fatima as it talks about the Robe of Light of the Bishop that was seen at the vision of Sister Lucia, seeing a Bishop Dressed in White or a Robe in Light is the clear connection. Francis in Fatima: "I am the bishop dressed in white."

Now, why add the Mystery of Light?

Our Lady of Fatima apparition, speaks about a war the Children of Light vs. the Children of Darkness. Genesis3:15

If there is a war destined to happen, what would be the weapon of the Children of the Light?

St. Padre Pio said, "The rosary is the weapon in the end times."

The battle would be between forces of Light & forces of darkness, and St.John Paul II like St. Dominic was handed a weapon to fight the evil forces in their days.

St. John Paul II was inspired to add the Mystery of Light as a weapon in these end times.

Sister Agnes Sasagawa said, "the only weapon left to us in the end times , will be the rosary."

The Mystery of Lights was inspired and given by Our Lady to St. John Paul II Totus Tuus.

Light vs. darkness.

"The light shines in the darkness and the darkness did not overcome it. "(John1:5)

"In the end My Immaculate Heart will triumph." (Our Lady of Fatima)

THE MESSAGE OF FATIMA

It came from Blessed Bartolo Longo.

And this practice of Bl. Bartolo inspired St. John Paul II to add the Luminous Mysteries to the Rosary. Regarding the influence of Bl. Bartolo, JPII, in his letter on the Rosary writes:

In the spiritual journey of the Rosary, based on the constant contemplation – in Mary's company – of the face of Christ, this demanding ideal of being conformed to him is pursued through an association which could be described in terms of friendship. We are thereby enabled to enter naturally into Christ's life and as it were to share his deepest feelings. In this regard Blessed Bartolo Longo has written:

“Just as two friends, frequently in each other's company, tend to develop similar habits, so too, by holding conversation with Jesus and Mary, by meditating on the mysteries of the Rosary and by living the same life in Holy Communion, (that is, in friendship with them) we can become, to the extent of our openness, similar to them and can learn from these supreme models a life of humility, poverty, hiddenness, patience and perfection”.

Rosarium 15

Clearly, St John Paul II would not have been the first to think of the idea that public life of Jesus had not been adequately represented in the traditional Rosary. I remember to have heard it from a humble Carmelite priest of Kerala, Southern India in 1983, with the exception that the priest had proposed inclusion of the resurrection of Lazarus as one of the mysteries. The fact is that St John Paul II , being the Pope, was in a position to put into practice what he believed, or had been influenced into believing by the loud thoughts of someone else, as a fitting addition to the traditional Rosary.