Because she's too young. Climbing at these altitudes isn't just some physical exercise, it takes serious toll on one's body, under severe strain and dangers. The lack of oxygen and low pressures literally eat away at your organs and muscles. There is a high risk of pulmonary embolism, pulmonary edema, cerebral edema, hypoxia, hypothermia, frostbite, etc. All these are potential killers which have claimed many very renowned climbers. Now with kids, who have much weaker bodies and a lot less strength, these risks increase exponentially. On top of that (given that Everest is a relatively easy climb) there are more than acceptable physical hazardous present which no kid should be allowed to face. We have age limits everywhere for a reason, right?
Every year some selfish, impotent parent decides to risk their poor child's life for vicarious glory, which is pathetic. And every time you hear an uproar of disgust from the mountaineering community.
The Nepalese authorities do not allow anyone below 18 years (maybe it was 16) of age to climb and because of that the above mentioned pitiful parents take their kids up from the Chinese side where there are no age restrictions. At least this is how it used to be the last time I checked and hence my question.
All this, of course, doesn't take anything away from that hardy kid's achievement.