The main reason is because that land had not been colozinized by a major power. This map shows the extent of colonization of the Americas at the start of King Philip's War, two years after the Marquette and Jolliet expedition. As the map shows, the Mississippi River was already used for trading, but since there was no need to go to Minnesota, why go there? It was dangerous (natives) and the nearest territory was in Canada and Michigan. This map shows how the farthest exploration had been into southern Wisconsin before offshooting (incidentally at the same latitude as the end of the river, but that's not the point). This website details how they got there and why (The Northwest Passage).

Sorry if this might not have answered your question.