Why are armors ranked highly on tier lists?

Because tier lists (or at least the tier list here) is based on arena viability and armored units are in the highest BST bracket and therefore can achieve the highest score.

High base stat totals to get into/ stay in tier 20 in arena, takes less damage/ more durable compared to other units, Bold Fighter and Vengeful Fighter armor only skills, keeps your beers icy cold and refrigerated

Because they have things like Vengeful Fighter and Bold Fighter available