I wonder why they didn't take advantage of the fact that it could draw a huge amount of lines in a small amount of time and tweak their edge-detection algorithm to draw more lines.
Anyway, I'm sure there was very good reason why they haven't... likely the lines they can draw can't be very fine with the sort of marker they have to use.
- The drawing is sent to the Arduino as a block, then drawn, so it has to fit in the Arduino's RAM. More complex drawings would require streaming the data during the drawing. That's totally possible, but it is more work.
- We quite liked the way these severely-simplified sketches came out. Because the edge-finding and vector-simplifying code is quite stupid, the best pictures tend to be the simplest ones, and doing more lines might well just make the picture more noisy.