That's a pretty good map for its time. The cartographers probably had access to Spanish maps which provided the details of Mexico, the American southwest, and the California coast up to "Drakes Harbor" (now known as San Francisco Bay); and Russian maps showing Siberia, the Bering Sea, and the Alaskan, Canadian, and northern California coasts. Not many people know that the Russians had a trading post at Ft. Ross on the California coast.
The lack of detail for the American and Canadian West is probably due to the lack of any major exploration at that time, and the lack of large rivers in the Plains States. The Indians didn't do maps, and the fur trade was still a few years in the future. After the Lewis and Clark expedition in 1804, much of the Louisiana Purchase was thought to be a useless wasteland. Thomas Jefferson thought that it might take a thousand years to fully explore and populate the area. Instead, the fur trade resulted in major exploration of the mountains, and a desire to own land got the region populated in less than 100 years.