Catholics have a special relationship with Our Lady that most other denominations do not have. Because of this, we have been blessed with apparitions where she gives warnings and messages. Why aren’t these messages given to other christians for their safety and salvation also?
All public revelation ceased with the death of the last apostle. While it is possible for our Blessed Mother to appear here on earth, she will not “add” to what we already know we need for salvation.
The way of teaching that Christ established is through his Church, not through apparitions. If the Church determines that an apparition is authentic and that there is something that needs to be taught, the Church will then teach us. But it will NOT be an article of faith that was never revealed before. We are not burdened with the responsibility of determining which reported apparition is authentic and if what is being said is consistent with our faith. That is the Church’s responsibility and that is such a blessing.