If you have to 'cut to the chase', Here's my three best reasons:
In no particular order:
PLURALISM. That's the concept that Heaven welcomes all religions. It might be hard to accept that your Protestant friends might not go to Heaven, but that's the Dogma position of the Roman Catholic Church, and therefore one that the Blessed Mother is not likely to endorse.
THE APPARITION OF A SNAKE, and SATAN. At the first apparition, Vicka asked it that was a snake. She was told that it wasn't, but the children all ran away. On January 10, 1982, Mirjana tells of a strange apparition where from the light which normally precedes the Virgin's arrival, SATAN EMERGED, HALF DIABOLICAL, HALF DISGUISED UNDER THE FEATURES OF MARY.
THE VISIONARIES NEVER SAW THE BLESSED MOTHER's BARE FEET. At all of the other Marian Appiritions, like La Salette, Lourdes, Fatima, and Padre Pio, the Visionaries could see Our Mother's bare feet below her dress. All of the Medjugorje Visionaries have reported that they were never able to see Our Blessed Mother's feet, at any time.
Who cares about the Blessed Mother's feet?
Saint Padre Pio, who was visted several times by Our Lady, immediately recognized, on one occasion, that that particular appirition was false, because he could not see the bare feet of Our Lady, below her dress. He knew that the Devil has many powers, but he cannot change the appearance of his cloven hoofs. He immediately told the Devil to 'Be-Gone', and the Devil vanished!