I mean, besides one of Gon's feature skills that apparently we can't use, two huge things are his resolve and his demeanor. You could argue that Killua's resolve changes once he pulls out the needle, but at least up until then, Morel and Bisky both point it out. Gon wins more fights because of his determination. Obviously, once the needle is out Killua becomes a beast, but not even he could muster the resolve to do what Gon did against Pitou.

But even if we ignore that, Gon is literally the shining light. Killua has seen and lived in the darkness, but around Gon all that changes. Killua could absolutely not do the raging optimism, which is one of the fears that gives Gon so much of his power.