Why? In raid instances, trash is not worth mentioning. One thing is that trash often includes a boss ability in any form (so one can see those abilities before facing the boss) and clearing it is in no way part of the challenge. Wiping to trash may happen but no raid group has ever stopped a raid event because "Let's stop here, the trash is too hard!"
On the other hand, trash in mythic+ dungeons ARE a threat, especially on fortified weeks. Wiping to a trash group can result in an expired timer. To understand trash in a dungeon, we should add their abilites to the dungeon journal.