Because the Blessed Virgin offered them her hand

In the Franciscan chronicles it is related of Brother Leo, that he once saw a red ladder, upon which Jesus Christ was standing, and a white one, upon which stood his holy mother.

He saw persons attempting to ascend the red ladder; they ascended a few steps and then fell; they ascended again, and again fell.

Then they were exhorted to ascend the white ladder, and on that he saw them succeed, for the Blessed Virgin offered them her hand, and they arrived in that manner safe in paradise.

Saint Alphonsus of Liguori

In The Glories of Mary