The main reason napoleon divorced josephine and married marie louise of austria was because

Because he wanted to have an heir.

Explanation:

That was his main reason. Josephine was believed to have become barren, though there is no certainty in this regard. She had children from a previous marriage, but she never had a child with Napoleon. But his decision was also motivated politically. By marring an Austrian empress, Napoleon made an alliance with the House of Austria, with the Habsburgs, an important move given the fact Austria was a major power in the 18th and 19th century.

Napoleon wanted a male heir to his throne so he divorced Josephine since she can't give her any children, let alone a son.

Explanation:

The marriage of Napoleon and Josephine was a match of love. But after a few years of marriage, it was pretty much evident that she wasn't able to conceive. So, even though he still loved her, he began to look for someone else who has the possibility of providing him an heir. And Josephine also agreed to the divorce even though she also still loved him for she also want him to have an heir.

Napoleon then married Marie-Louise in the hopes of getting an heir and also validating his empire. Thus, by marrying her, he was also eventually taken in by the German imperial royal family, as Marie-Louise was the Archduchess of Austria. And with her, they got a son who later succeeded him briefly as Napoleon II.