And look at this awesome quote:
"By the Force! I thought a lightsaber could cut through anything. The walls are barely scratched. The only thing that can resist a lightsaber is…Mandalorian iron!" ―Exar Kun, regarding Freedon Nadd's tomb
Beskar was nearly indestructable. Exar Kun had trouble cutting through a door, I don't think he'll be slashing through a Mandalorian army anytime soon. If Exar Kun's lightsaber can't cut through Mandalorian armor, what makes his other forces able to defeat Mandos?
Beskar is used from ships as well. Mandalore's Army has MUCH better armor, and, with their war machines (also with Beskar) I don't see how Kun's army would be able to take them down.