The Moselle does not run straight. It meanders through a slate massif in a series of tight loops, so the distance a ship travels is far greater than the distance it advances, and the bends themselves limit how large and how fast a vessel can comfortably work the river.
Those loops are also the reason for the vineyards. The river's curves create south-facing slopes above the water, which is what makes steep-slope viticulture possible this far north. The landscape you look at from the deck and the wine the towns are known for are the same geographic fact seen twice.
For a passenger the practical effect is scale. Towns sit directly on the water at the foot of the slopes, moorings are in the middle of them, and a day's sailing covers a modest distance. The Moselle is usually combined with the Rhine rather than sailed alone, because on its own it is a short corridor.