Above St. Louis the Mississippi is a managed navigation channel: a series of locks and dams holds the river in a stepped chain of pools deep enough for barge traffic. Below it, the river runs free of locks all the way to the Gulf, wide and heavy with a strong current and shifting sandbars.
That difference changes the sailing. On the upper river the vessel works through lock chambers between bluff-lined pools and small towns; on the lower river it makes long, uninterrupted runs between widely spaced landings, with hours of levee, forest and open water in between.
It also explains the vessels and the landings. Many are paddlewheel-style boats that go alongside simple landings rather than into ports, because much of the corridor has no cruise infrastructure at all — the towns face the levee, not a terminal. Long voyages are usually one-way, since the corridor runs thousands of kilometres in a straight-ish line and there is nothing to be gained by retracing it.