Compare:
A = sort(randi([-2 9], 300, 500));
surf(A)
title('edgecolor default')
figure
surf(A, 'edgecolor', 'none')
title('edgecolor none')
Notice that the first of the two surface plots is nearly completely black, but the second of them, with edgecolor none, looks fine.
The difference is that in the first one, all the edges have been drawn in black. But edges have fixed drawing width: if you draw an edge and then zoom the plot in or out, the edge stays the same thickness on the screen. When you have enough data that the view has to zoom out to look at it all, then the data coordinates get squished together compared to physical drawing coordinates, so the data coordinates at which the edges get drawn get closer together, but the width of the edges stays the same. At some point, the edges are pretty much touching, and all you can see is the edges.
