I like to think that its because both BLACK and RX's Kick are not strong enough to kill *any creatures/monsters that are higher (more important) than the usual, standard monsters of the week. E.g in BLACK, while the Rider Kick usually finishes off Gorgom monsters, it still couldn't make guys like Shadow Moon and Birugenia go kaboom (and BLACK had Rider Kicked Birugenia on at least three different occasions). And as we know, the RX Kick is useless as a finisher.
So... the combined Kick effort of both BLACK and RX couldn't make a big character like Geist explode so easily.
*except the times when he managed to make it work against 2/3 of the Gorgom Daikaijin.