Because it's a relevant fact.

edit: By the way I find it very sexist they mentioned he was a boy.

He scrawled his name in Chinese, didn't he? It'd be rather pointless to avoid.

Young boy defaces ancient Egyptian sculpture, world cries out for manner.

Because "human" defaces ancient Egyptian sculpture, prompts online outrage...

...doesn't sound the same.

Best guess: Ethnic information adds flavor to the story, sparking curiosity that could lead to more hits.