Because, his uncle is Crown Prince Philippe; not his mother (i.e. His mother is the second born child, and therefore wasn’t bestowed the title Crown Princess like the first born is). Crown Prince Philippe's first child, Princess Elisabeth, will become Queen when Philippe dies or abdicates, because of the succession lines. If something happens to Elisabeth and is unable to fulfil her duty as Queen before she produces an heir, Gabriel (Elisabeth’s younger brother) will then become king. If something happens to him, Astrid, if she is still alive, will become Queen. If she is not, then Amedeo will become king, and the chances of that are highly unlikely. I can't really explain it that well, but I hope that helps. Maybe someone can explain it better.