Why an armored cruiser's armour was so thick

Wasn't that armour thickness excessive for a cruiser?

Not for an armored cruiser of this era. The type stood immediately below battleships in the contemporary status hierarchy.

A viewer remarks that the class's armour seems unusually heavy for a cruiser. These were armored cruisers, a category immediately below battleships in the period's hierarchy and far above protected cruisers, scout cruisers, torpedo boats, and destroyers. On foreign stations they were large, powerful symbols of naval status. A modern cruiser label can obscure how formidable the class appeared at the time.