Because my setup is unbelievably heavy. Contrary to what people say, friction sliders are actually easier to control lighter DSLR setups with. With ball bearing sliders you are using mostly your hands to control it, which with lighter setups can make it shakier. With a friction slider such as this you rely less on your hands and more on the resistance of the track to keep it smooth. If you are using a more simple setup such as just the DSLR + lens + maybe VideoMic, then this is a great choice. However when I mount my 501HDV + DSLR + Tokina + Preamp + NTG-2 + XLR cables it's just simply too heavy to move smoothly. When I made that video above I was using the aforementioned lighter setup, which worked just fine.