Gimli was unaware of Balin's death because there was no communication between Balin's colony in Moria and the Dwarves of Erebor (the Lonely Mountain) where Gimli resided.
The Isolation of Moria
When Balin led a group of Dwarves to reclaim Moria (Khazad-dûm) nearly thirty years before the events of The Fellowship of the Ring, their efforts initially met with some success. However, their triumph was short-lived. The ancient Dwarf-realm was still fraught with dangers, including Orcs and other creatures that had festered within its deep halls.
- No Contact: Once Balin's expedition entered Moria and established their settlement, they lost all contact with their kin in Erebor. Travel through the wild lands of Middle-earth was perilous, and maintaining a secure line of communication across such vast and dangerous distances was virtually impossible.
- Sealed Fates: The colony ultimately faced a tragic end, being overwhelmed by the forces of evil dwelling within the mountains. The full details of their demise were unknown to the outside world, including Gimli and the other Dwarves of the Lonely Mountain. They held out hope that Balin and his followers were thriving, despite the lack of news.
The Discovery in the Chamber of Mazarbul
The grim truth of Balin's fate was only discovered by the Fellowship of the Ring when they passed through Moria.
- The Great Book: In the Chamber of Mazarbul, the Fellowship found a large, fragmented tome – a record book of Balin's colony. This tome documented the history of their attempt to reclaim Moria, detailing their initial successes and eventual downfall. It recorded the final moments and the grim end of Balin himself and his entire company.
- Gandalf's Revelation: It was through reading this mournful account in the "Book of Mazarbul" that Gandalf, and by extension the entire Fellowship including Gimli, learned the definitive truth of Balin's death and the destruction of his colony. Before this moment, Gimli had been filled with hope and excitement at the prospect of finding his kinsman alive and well.
In summary, the lack of any means of communication between the besieged colony in Moria and the outside world meant that Gimli and his people in Erebor had no way of knowing the tragic events that had unfolded within the ancient mines.
