Here's why:
* The Map's History: The map was a treasured family heirloom, passed down through generations of the Durin line. It was originally drawn during the height of the dwarves' prosperity in the Lonely Mountain.
* Thror's Connection: Thror was the last king of the dwarves who resided in the Lonely Mountain before Smaug's arrival. The map likely served as a guide to the mountain's secrets and riches, knowledge passed on to his descendants.
* The Map's Importance: The map played a pivotal role in the story, guiding Thorin and his company on their quest to reclaim their ancestral home and defeat Smaug.
Therefore, while Thorin Oakenshield used the map, it was created by his ancestor, Thror.