We need to consider the Beowulf poem as both a well-crafted expression of literature and as a path to understand the underpinning oral tradition. Taking the last episode/monster first: the dragon episode may or may not have been consistently linked with the Beowulf character in the oral tradition. Things can be fairly fluid in folklore, so it is easy to imagine this episode standing alone, associated with any number of heroic figures and at the same time appending itself to a great hero such as Beowulf. The point of the episode in literature is to provide the old king with a good death. The center of gravity is on the king and his death; the dragon is merely a vehicle. It is also a distinctly separate episode from the first two thirds of the poem, so it is not surprising that the adversary is treated differently.

Turning to the Grendel/Grendel's mama episode, we see the bulk of the poem and what appears to have been the core of the original oral tradition. As this story churned away in folklore, it is easy to imagine that Grendel may not have consistently died in the original struggle but rather retreated to his underground/underwater lair to be confronted again by the hero. This can only be pure speculation, but oral tradition can float along these lines. The folktale that manifests much of this same story, classified as Aarne-Thompson 301, "The Bear's Son Tale," often features a single monster confronted above and below ground. Regardless, in the poem there is no question that the mother is of less literary focus/importance than Grendel. As the user with the utterly charming name /u/randomasfuuck27 eloquently points out, neither the mother nor the dragon was "around long enough to be named."

Grendel is clearly the focus of the literary poem and in all likelihood the underlying oral tradition. It is no accident that the Anglo-Saxon term "aglǣca" is used for both Beowulf and Grendel. The two are both "fierce fighters," equal combatants and the true test of one another's might. Defeating Grendel is the proof of Beowulf's heroic strength. The remainder of the poem merely adds to a structure established by the Grendel episode. The Grendel episode is the literary center of gravity - and that would have been true whether considering the poem or the cycle of folklore that carried the story through most of its existence, in the oral tradition.

Some of this answer rests, perhaps, too heavily in the realm of literary criticism (which my Anglo-Saxon instructor once called "an open door to phonyism"). That as it may be - and in spite of the fact that we are left with a great deal of conjecture about what the original oral tradition may have looked like - certain conclusions seem reasonable. And considering the structure of the poem, we see that Grendel, who is so central to the poem and dominates so many of its lines, should be the creature who is given a name and a vivid identity.

For sources, consider the classic sources R. W. Chambers, Beowulf, An Introduction to the Study of the Poem (1921) and F. Panzer, Beowulf (1909).

edit: I see that post by /u/randomasfuuck27 has been deleted, but he/she had a good point, which survives with my comment.

Grendel may be related to Old English gryndal "fierce, angry" and Old Norse grindill "storm, wind"; it may also be derived from the Old English verb grindan "to grind, destroy".

http://books.google.com/books?id=6C4W4T-6VcYC&pg=PA206#v=onepage&q&f=false

http://books.google.com/books?id=1djfAgAAQBAJ&pg=PA135#v=onepage&q&f=false

Follow up question - should we take "Grendel" to be the creature's NAME, or merely what the Danes called it? From that point of view, all the creatures had names, "dragon", "mother" and "grendel".

If it's actually the creature's name somehow, that's an intriguing bit of omniscience for the narrator.

I think there is intended symbolism from naming her simply Grendel's mother. Naming the mother something else would have created a new monster lineage. The story is complete as the protagonist kills both the beast and the source of evil.