Can't change dungeon difficulty setting?

Hi, I didn't know where else to put this question. I'm soloing The Forge of Souls, Pit of Saron, and the Halls of Reflection in order to get Battered Hilt for the quest line and eventually for the sword for some transmogrification. Anyway, the Battered Hilt only drops in heroic mode of these instances, and when I click on my character portrait to change the dungeon difficulty, the "Dungeon Difficulty" selection is grayed out, and I can't click on it. I haven't done these dungeons before. Just wondering why I can't change the mode to heroic, thanks for the help.