Taking the Earth as example: paper making was first recorded in China since the 1st century, but only made its way to Europe in 11th century. Astral navigation also existed since ancient time, yet only until the Age of Discovery (15th century) people started the extensive overseas exploration around the world, all due to European countries pushing for colonization. That's how it is with technological advancement: they happened because people have the want & need (or suitable circumstance), not because xxx amount of time has passed.
For a millennium after the collapse of the Empire, the Shantean kingdoms had been gradually losing ground to the bloodthirsty zealots, while internally stagnated because of the witches terrorizing society as a whole. It's no wonder that they couldn't develop anything substantially. Other countries such as not-Britain have superior shipbuilding and navigation technology. Even then, long-term seafaring is no easy task, without a solid reason (big money talking, or a desperate situation) nary anybody would risk their life for it.