Why is Grendel so alone? Why has Gardner written him as seemingly the only conscious creature in his family or cave?

It's not easy being the descendant of Cain living with your mother in a swamp cave. Really Grendel is in awe of his own power but like many a bully, terribly alone,

"I was Grendel, Ruiner of Meadhalls, Wrecker of Kings!

But also, as never before, I was alone." (80)

Grendel makes this miserable statement when he discovers the dragon's enchantment has left him invulnerable to harm from the Scylding's weapons. He glories in his power, but realizes that his impervious hide now separates him even more from the world of mortal men. In a flash of insight, Grendel recognizes that his condition has not improved, but has instead become much more hopeless as he seeks a place in this world.