Because LOTR isn't the first time Balrogs are mentioned, and in no other instance in the legendarium do they have wings, much less wings so big as to cover the entire surface of a wide chamber of Moria.
It would have been very advantageous for Balrogs to have flight, yet they never fly. It is much more likely that the passage in LOTR refers to the darkness emanating from the Balrog filling the chamber. This would be consistent with other evil creatures in this world, of whom darkness emanates and envelops those around them.
TheBerraExperience t1_j3ru0hz wrote
Reply to comment by Bojikthe8th in Balrog of Morgoth, Me, Ink and Acrylic, 2023 by GoblinSuplex
Because LOTR isn't the first time Balrogs are mentioned, and in no other instance in the legendarium do they have wings, much less wings so big as to cover the entire surface of a wide chamber of Moria.
It would have been very advantageous for Balrogs to have flight, yet they never fly. It is much more likely that the passage in LOTR refers to the darkness emanating from the Balrog filling the chamber. This would be consistent with other evil creatures in this world, of whom darkness emanates and envelops those around them.