Because I believe most righthanded gamers would rest their indexfinger on X and their middle finger on Z, and since you (in most cases) tap quicker with your indexfinger than your middlefinger it is more logical to place the shoot button on the quickest finger. Or something like that.
Yeah, if you use Z/X make sure you give Y the same function as Z since you get a lot of players from such as Germany.
As for the button functions, it depends which is the more crucial action. If the player can hold down the button for rapid fire, I suggest Z as shoot and X as jump. Ditto if there're generally going to be a lot more jumpy bits than shooty bits. If jumping is less common and shooting takes a front seat, I'd shoot with X and jump with Z.
Or just make it configurable. Or even have the actions duplicated on QW or AS, with the functions reversed.