Lewis & Clark's team wasn't the first attempt by the young United States to explore the territory west of the Mississippi.

Lewis and Clark and their Corps of Discovery were NOT the first group sent out to explore what is now the northwestern United States for the young nation. They were only the most successful ones to travel over land and several rivers from St Louis on the Mississippi over to the mouth of the Columbia River at the Pacific Ocean.
There had been three other atempts by the young nation to discover what was in this region. This at the same time England, France, and Spain, who owned the land west of the Mississippi, were looking at it for their own uses.
In fact the very first successful trip was in what is now parts of Canada by Alexander MacKenzine and Alexander MacKay in 1793 while looking for an easier way to move their furs from their inland trapping sites. In fact it was in the publishing of their book that urged President Thomas Jefferson to finally meet his goal of exploring west of the Mississippi. However, before the successful attempt by Lewis and Clark and their men there were three aborted attempts by the young nation.
The first attempt across the continent was by Lieutenant Armstrong in 1790 under the leadership of Secretary of War Henry Knox. Secretary Knox greatly underestimated the challenges and supplies need for such a venture. Lieutenant Armstrong and his men got no farther than the St Louis settlement before they had to turn back.
The next overland attempt was sponsored by the American Philosophical Society of Philadephia, of which Thomas Jefferson was a member. When they had gathered enough money together they hired French botanist, Andre Michaux. Part of his instructions dated 30 April 1793 were “. . . to find the shortest and most convenient route of communication between the U.S. and the Pacific Ocean within the temperate latitudes . . .” and that “. . . It has therefore been declared as a fundamental object of the subsription (not to be dispensed with) that this river (the Missouri) should be considered and used.”
Michaux received the job of exploring the west primarily because of his training as a scientist. What the Philosphical Society had not known was he was a loyal Frenchman. When Thomas Jefferson and the other members of the Society discovered he worked as a secret agent for the French Republic and that his goals for them were warlike toward the Spanish lands west of the Mississippi they had him recalled in Kentucky by the French government.
While this was another set back for the desire to know what was on the other half of the continent, Thomas Jefferson did not give up on his goal of having a single coutry from the Atlantic to the Pacific Oceans. He realized it would take patience and perseverance to reach his goals, he could wait; besides other parts of his life were calling him. It would ten years, time for a lot of events that would affect the young nation to happen.
By the time the Corps of Discovery, with Lewis and Clark as their co-leaders, finally went out it was a commission from the government with government funding. President Thomas Jefferson saw his dream turn into a reality. Many things lead to the success of this particular trip: the combination of leadership, teamwork, and the fact that all the men were frontiersmen both at heart and in their bodies. It became a success not only because they found a waterway from the Mississippi River to the Pacific Ocean but because they brought back valuable information on the flora, fauna and Indian tribes that would impact the young United States for decades to come.