why CO2 is not organic
Carbon dioxide 
"is" neither organic nor inorganic, it's just carbon dioxide. 
This distinction is nothing but a convention to separate things 
between the textbooks and professors. 
Carbon dioxide forms carbonic acid in water and the usual salts. 
This was/is reason to think of it as inorganic. 
On the other hand, urea, the diamide of carbonic acid was thought
to be organic when Wöhler synthesized urea for the first time 
in lab, today it is categorized as inorganic.