Reason could be Aluminum is very reactive and never found in pure form on earth. Ithas somewhat nonmetal behaviour and react with alkalis to form aluminate. Further, it has low density. read more
aluminum "Although aluminum is the most abundant metal in the earth's crust, it is never found free in nature. All of the earth's aluminum has combined with other elements to form compounds. read more
Earth crust contains aluminum in abundant amount over the continental region while earth crust contains magnesium in abundant amount over the ocean region. Deep down to the earth's surface, in the core of out planet, iron is the most abundant metal making it gravitationally active. So it's all about the region you are considering. read more