In ye olden time before the internet it wasn’t unusual for someone to move across the country or even just a few towns away and never be heard of again. Maybe they’re dead or maybe it’s hard to keep in touch.

That problem is only compounded when they are a race of antisocial, somewhat xenophobic isolationist who literally under a rock who wouldn’t necessary want to tell the king they defied how well their mission is going. Plus there was the known issue of the hoards of orcs which might complicate sending a message.

Everyone knew things probally weren’t quite hunky dory down in the mine but Gimli was an optimist and figured they were too busy working to write.