because the first hokage was like a freak of nature =/
he was the only one (so far, not sure if the soap's second son could do it) who could use it... not even his own brother has that kg... but then again... his brother, the second hokage was a specialist with the water element, being able to summon large quatity of water and perform jutsus with few seals =/
i think its cause she doesnt have the right elements to create mokuton