My assumption is that Coerthas was cold enough that Miqo'te didn't travel there plus their hostilities to foreigners made it very hard for Miqo'te to have been their longer than just as adventurers or travelers. Even considering the Calamity.
After the Calamity, they've all probably left because of the changes, ie too cold for Seekers too clouding for Keepers.
After the Calamity, they've all probably left because of the changes, ie too cold for Seekers too clouding for Keepers.