Because Atlantis supported the Allies during WWII.

Not only did they provide troops and exceptionally advanced, primarily undersea military equipment, but they provided the Allies with a symbol of inspiring beauty:

Aquaman's occasional Queen and constant companion, Mera. 

Our Mera DC Bombshells Gone Fishing Men's T-Shirt sports a stunning image of Mera rendered as a Bombshell pin-up girl -- an attractive morale-booster applied to the sides of bombers, tanks, aircraft carriers, and the occasional milk bottle.

Made from 100% cotton, this Mera t-shirt for men is rendered in a clear sea-blue, proving that Aqua-women were more effective at inspiring victory than rugged Aqua-men.