Quoted By:
Because it has more than 70 BST than the other Fossil Pokemon (50 in the outlier Aerodactyl's case). They wanted to make a fast glass cannon Fossil, but a 495 BST doesn't lend itself well to that without intense min-maxing. So they upped the BST to 567 and gave it a debilitating ability to balance it out, especially since Archen and Archeops is ridiculously good for when you are able to get one. It's an in-game powerhouse.
