Command over voice, as over hand.
See Arthur L. Guptill’s classic Rendering in Pen and Ink:
…In writing, a comparatively small number of standardized curves and straight lines are combined in a methodical and frequently repeated manner.
In drawing, there is almost no end to the variety in length, direction, and character of the lines used or to the methods of combining them. The penman, then, seeks a certain monotonous perfection of stroke; the artist, on the other hand, must acquire the greatest possible versatility in the command of his instrument. The artist must be able to draw long, sweeping strokes, bold vigorous lines, crisp dashes, and delicate dots. He must be able to draw reasonably straight lines and pleasingly curved lines, singly or in combination. And he must have the skill to draw all of these when and where he pleases on his paper, vertically, horizontally, or slantwise. This must be accomplished, however, with little conscious effort, so that attention is freed for the development of the composition as a whole.
Compare the human hand against the human voice.
In natural (or even excited) speech, there are only so many things you could do with the voice. (Unless you want to sound silly.)