Because of the length... when the shafts get to long you have a number of issues to deal with. Harmonics, twisting, and weight.
Long tail shafts require thicker materials to return the same strength (both torsionally and longitudinally) which adds weight to the rotating mass. This can cause vibrations and actually eats up power to turn the shaft. Splitting the shaft allows using a thinner, lighter material and smaller u-joints. This reduces rotating mass and actually makes for a less vibration prone installation.