Why is outlined color the most popular style of illustration used for comic strips, captioned cartoons and other funny stuff?
Could it be that our mind's eye is more used to seeing funny stuff in this style than in any other? Or, that line and color is simply funnier to look at? My best guess is that it's a combination of both. In recent history, we have all have grown accustomed to the look itself as being an indicator that a message may be humorous. And whenever black line is applied to anything, even a photograph, the result is a degree of change that usually favors the lighter side of things.
While Leonardo would never have considered such a style for the Mona we know, its use has saved tons of rendering time over the centuries. Which may be another reason the technique, in its many forms, has survived for so long. After all, the quicker a good laugh can be created, the more joy there is to go around.