Allah can do anything, even break the moon in half. If he can do anything, how can we limit his powers to comply to the rules of this universe that he created.
If he wanted, only a few people will see the event or everyone will.
Ghamidi Sahib provided very strong evidence that the event did happen.
Why not the whole world saw it?. 1. The world has a day on one side and night on the other. Also, moon is not visible all the time all over the world. There are also cloudy or foggy conditions. Also someone has to be looking at the moon to notice the split.
Then at that time World population was much less and there were no methods of instant communication or to video tape an event. So how would the news spread around the World if someone saw it? If someone told others that he saw such event, other people will not believe it. Soon the event will be forgotten.