First of all, I’d be wary of saying that Mary is “still appearing,” as that wording implies that there are approved apparitions happening on a daily basis. Though alleged Marian sightings do make for great news stories, only a very few of them have been investigated by the Church and found to be worthy of belief. It’s also important to remember that even the handful of apparitions that are approved of by the Church fit into the category of “private revelation,” meaning that Catholics are not required to believe in them (this is distinct from “public revelation,” such as the teachings of Christ.)
That said, when you look at the apparitions that have been found worthy of belief, common themes emerge. Many of them involve Mary asking for prayers and personal repentance (as at Fatima, Portugal, in 1917). Others lead to physical healing (we see this in the miraculous spring at Lourdes, France). The apparitions of La Salette, France in 1846 and of Kibeho, Rwanda in the 1980s were prophetic in nature. One common thread connecting nearly every recognized apparition is that the people to whom Mary appears are young, poor, or in some way marginalized by society. In this way, Marian apparitions serve to remind us that the “least among us” are holy and deserving of dignity.
Though it’s hard to fully answer the question of “why” Mary has appeared on earth, the character of the apparitions point to a mother who desires peace and virtue among her children. Perhaps she senses that some of us need a little extra convincing or a personal “visit from Mom” in order to make that happen.