The reason, of course, is that not everyone has the ability to scroll. Personally, I use a laptop as my primary machine and only have the touchpad as my pointing device. There is a way to scroll with the touchpad, but it's incredibly painful to use, and is very hit or miss on my machine. Also, I don't know how common they are today since I haven't bought one in years, but I'm sure there are users that are still using old mice without scrollers.
That said, very nice site. I can see it being very useful to me in the near future.