I think vertical scrolling worked well in that case because it was used to display depth. The information you are displaying is literally about going down. Horizontal scrolling feels more natural to me here because the point is to show how empty and vast the area between planets is.
The downside is that it's an experience that admittedly only works if you're using a trackpad, or a mouse with good side-scrolling support. So people with laptops / trackpad mice are probably okay.
A little known fact seems to be that if you hold Option (or was it command?) on OS X while using the mouse wheel, you scroll horizontally instead of vertically. Found that out by accident.
The downside is that it's an experience that admittedly only works if you're using a trackpad, or a mouse with good side-scrolling support. So people with laptops / trackpad mice are probably okay.