From the wiki

Sentinels are strongly defensive characters and excel at locking down enemy generals and holding the line. Expertise is the Sentinel's key attribute, aiding their melee evasion chance in battle and reducing construction costs in any commandery that are assigned to administrate. The key expertise-based skills in their skills tree improve their melee evasion, ranged block chance, and melee damage capabilities. Sentinels work well when grouped with polearm and melee infantry.

Sentinels are not tank in the traditional sense; you can use them as tanks because they have a very high melee evasion meaning they can be hit very rarely by the common grunts. And such a combat-style is in line with how female fighters used to fight, they're not the big brutes that go around rampagin in the midst of a company of men and slaying them left and right.

All the while, still from the wiki

Champions are duelists. These guys excel at taking out enemy heroes.

Vanguards are the unit killers, and they're great at cutting their way through tons of common soldiers.

As I said previously I believe it's a matter of keeping things in line with how they worked historically.