Because they're amazing. The Golden Book of Facts and Figures by Bertha Morris Parker, in a section on money, has a heading for "Pacific Islands (New Britain, San Cristobel)". The table below, describing items used as money and their relative value, reads (emphasis mine):
10 coconuts = 1 string white whales' teeth.
10 strings of white teeth = 1 string of red whales' teeth or 1 dog's tooth.
10 strings of red teeth = 50 porpoise teeth.
500 porpoise teeth = 1 wife of good quality.
1 "marble" (shell) ring = 1 good pig.
Stunning. Also, I like that the order implies that "1 good pig" is more valuable than "1 wife of good quality."