1. (Self-disclosure - I live in the western U.S.) What they didn't find[i is the most important aspect of the journey... that being there was [i]no] navigable waterway to the western coast. Most thought the upper Missouri River would surely lead all the way west. This caused the migration routes of the mid-1800's to focus on more southerly overland routes such as the Oregon Trail.
2. While Sakakawea (most commonly used spelling here... a huge reservoir on the Missouri River named thus) was certainly important, both Lewis and Clark were astute enough to have used Native guides. The Shoshones were critically situated though, living as they did in what became present day Montana and Idaho. The expedition over-wintered (December 21, 1804-April 06, 1805)
with the Hidatsa-Mandans near present day Bismarck, North Dakota and they would have provided good guides as would the later encounterd Nez-Perce.
The fact that Sakakawea had a baby at the beginning of the year in 1805 helped the natives be less concerned about any war like intentions of the expedition though. Sakakawea was fluent in most languages spoken along that route, although sign language was also universally in use.
3. Because the public at large viwed that part of the Louisiana Purchase as desert and totally worthless and of no value for settlement.
4. Why didn't you learn how to survive. They nearly perished every winter they were out.
5. Going south from their overwinter (1805-1806) in Ft. Clatsop (near present day Astoria, Oregon, would have been so much easier than the return trip they chose. This could possibly have been due to Sakakawea wanting to get back to her people, though.
And... why woasn't Charbonneau (Sakakawea's husband) more help in teaching you how to live off the land?