Cutting right to the chase: it's because the two unique abilities it has don't get much use.
Mantle of Inspiration only has an effect when something hits you specifically with a melee attack. Archers and casters don't care. Armor of Agathys and similar buffs cancel it(understandable but still). Worst of all, if you're playing well enemies simply don't get the opportunity to do melee attacks at all. You could use summons like mephits or Nimbus to bait attacks, but then you're making a plan to get hit instead of just killing everything.
Mantle of Majesty has little value if you don't have enemies that are already charmed. You either need to have someone waste an action charming enemies(when you could just kill them instead) or having a teammate with Mantle of Inspiration get themselves hit.
I spent my time in the patch 8 stress test goofing around, but when I'm actually trying? College of glamor? More like college of...self-defeating game mechanics! Am I right or am I right? Ok, but it really feels bad, as though any other bard could do it's job, but have more features to in situations other than mistakes.
