even if you only use your left thumb to hit space it's still faster than using , for your leader as the thumb really only hits the space key, command(if you are on a mac)/alt(windows) and maybe option/alt if you are on a mac. and it's natural to use the left keys after hitting space. whereas using any key on the right side of the keyboard after a leader press is awkward and slower because the , key is pressed with your middle finger. it's also a smaller target so it's harder to hit.