Return to Answer

If you go by the "Christianized" version of the story, the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).

...for in that place was part of the blood of our Lord Jesus Christ, that Joseph of Arimathea brought into this land, and there himself lay in that rich bed. And that was the same spear that Longius smote our Lord to the heart; and King Pellam was nigh of Joseph's kin, and that was the most worshipful man that lived in those days, and great pity it was of his hurt, for through that stroke, turned to great dole, tray and tene.

...

And therewith he [Galahad] kneeled down to-fore the table and made his prayers, and then suddenly his soul departed to Jesu Christ, and a great multitude of angels bare his soul up to heaven, that the two fellows might well behold it. Also the two fellows saw come from heaven an hand, but they saw not the body. And then it came right to the Vessel, and took it and the spear, and so bare it up to heaven. Sithen was there never man so hardy to say that he had seen the Sangreal.

(Malory, Thomas, Le Morte d'Arthur Part 1 and Part 2)

According to Mallory's version of the story (which is written after the Vulgate and post-Vulgate Cycles), the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).

...for in that place was part of the blood of our Lord Jesus Christ, that Joseph of Arimathea brought into this land, and there himself lay in that rich bed. And that was the same spear that Longius smote our Lord to the heart; and King Pellam was nigh of Joseph's kin, and that was the most worshipful man that lived in those days, and great pity it was of his hurt, for through that stroke, turned to great dole, tray and tene.

...

And therewith he [Galahad] kneeled down to-fore the table and made his prayers, and then suddenly his soul departed to Jesu Christ, and a great multitude of angels bare his soul up to heaven, that the two fellows might well behold it. Also the two fellows saw come from heaven an hand, but they saw not the body. And then it came right to the Vessel, and took it and the spear, and so bare it up to heaven. Sithen was there never man so hardy to say that he had seen the Sangreal.

(Malory, Thomas, Le Morte d'Arthur Part 1 and Part 2)

If you go by the "Christianized" version of the story, the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).

...for in that place was part of the blood of our Lord Jesus Christ, that Joseph of Arimathea brought into this land, and there himself lay in that rich bed. And that was the same spear that Longius smote our Lord to the heart; and King Pellam was nigh of Joseph's kin, and that was the most worshipful man that lived in those days, and great pity it was of his hurt, for through that stroke, turned to great dole, tray and tene.

...

And therewith he [Galahad] kneeled down to-fore the table and made his prayers, and then suddenly his soul departed to Jesu Christ, and a great multitude of angels bare his soul up to heaven, that the two fellows might well behold it. Also the two fellows saw come from heaven an hand, but they saw not the body. And then it came right to the Vessel, and took it and the spear, and so bare it up to heaven. Sithen was there never man so hardy to say that he had seen the Sangreal.

(Malory, Thomas, Le Morte d'Arthur Part 1 and Part 2

If you go by the "Christianized" version of the story, the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).

...for in that place was part of the blood of our Lord Jesus Christ, that Joseph of Arimathea brought into this land, and there himself lay in that rich bed. And that was the same spear that Longius smote our Lord to the heart; and King Pellam was nigh of Joseph's kin, and that was the most worshipful man that lived in those days, and great pity it was of his hurt, for through that stroke, turned to great dole, tray and tene.

...

And therewith he [Galahad] kneeled down to-fore the table and made his prayers, and then suddenly his soul departed to Jesu Christ, and a great multitude of angels bare his soul up to heaven, that the two fellows might well behold it. Also the two fellows saw come from heaven an hand, but they saw not the body. And then it came right to the Vessel, and took it and the spear, and so bare it up to heaven. Sithen was there never man so hardy to say that he had seen the Sangreal.

(Malory, Thomas, Le Morte d'Arthur Part 1 and Part 2)

There are many (many many many) different versions of the grail legend ¹, so there are two different ways of answering this.

If you go by the "Christianized" version of the story, the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).

(Malory, Thomas, Le Morte d'Arthur Part 1 and Part 2

If you go by the older version of the story, the quest for the holy grail represents an attempt to heal a king, which is connected to healing the kingdom (the health of the land is connected to the health of the king):

(a) There is a general consensus of evidence to the effect that the main object of the Quest is the restoration to health and vigour of a King suffering from infirmity caused by wounds, sickness, or old age;

(b) and whose infirmity, for some mysterious and unexplained reason, reacts disastrously upon his kingdom, either depriving it of vegetation, or exposing it to the ravages of war.

(c) In two cases it is definitely stated that the King will be restored to youthful vigour and beauty.

(d) In both cases where we find Gawain as the hero of the story, and in one connected with Perceval, the misfortune which has fallen upon the country is that of a prolonged drought, which has destroyed vegetation, and left the land Waste; the effect of the hero's question is to restore the waters to their channel, and render the land once more fertile.

(e) In three cases the misfortunes and wasting of the land are the result of war, and directly caused by the hero's failure to ask the question; we are not dealing with an antecedent condition. This, in my opinion, constitutes a marked difference between the two groups, which has not hitherto received the attention it deserves. One aim of our present investigation will be to determine which of these two forms should be considered the elder.

(Weston, Jessie, From Ritual to Romance². )

According to that (older) version, once the king/land is healed, the grail is no longer needed, and thus vanished.

What the grail represents is a highly debated question.

¹ The reason why the online encyclopedia cited in the question doesn't have this information is that online encyclopedias mix and match different versions of myths, without even thinking about where those versions came from. They aren't alone in this; most interpretations of myths (e.g. children's books, Percy Jackson) do this as well. Reading the original versions of myths is often more interesting than reading encyclopedias, as you learn the author's original intent and wording.
² This book is somewhat out of date, and the main theory in the book -- that the grail legend comes from religious rituals designed to make the land fertile -- has been disputed (to say the least), but this part of it's analysis is sound and supported by textual analysis.

If you go by the "Christianized" version of the story, the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).

(Malory, Thomas, Le Morte d'Arthur Part 1 and Part 2