There are more oxygen atoms in the reactants while there are less oxygen atoms in the product.
Both sides of the equation is supposed to be balanced for a balanced equation. If any one of them isn't balanced, the equation remains unbalanced.
The main reason why the reaction above can not be balanced is:
This chemical reaction SO2 + H2O -> H2SO2 is not correctly written.
It must be: SO2 + H2O -> H2SO3
hope this helps....
