Grendel telling the story makes readers feel sympathetic towards him . In the epic of Beowulf, Grendel was a monster who was pure evil but in this, he is almost childlike which makes the reader pity him. When he was being attacked by humans, "he bellowed for his mother” just like how a child would call out for his mom when he is hurt.
I think the reason Gardner chose to tell it from Grendel's point