The marks were placed my Louisoix for the player and people of 1.0 to pray to to help him seal in Bahamut. Ala Mhigo was (and still is) under imperial control and old sharlayan is currently innaccesable. The marks had to be reachable by players so the devsplaced them in existing places. Im sure theres Thaliak sigils all over old sharlayan.

Ala Mhigo on the other hand would have had Nymeia sigils if anything due to the mad king of ruin outlawing worship of any god but Nymeia.