why were women not allowed to vote until 1920 despite the women's rights convention in 1848 says they had the right to vote?

As you can plainly see, a convention in 1848 had little to no influence on the laws. Women's suffrage activism developed to serious consideration across many decades. It took a long time to even get enough women on board.

Bc women didnt have brains then and still dont now 

Because such gatherings do not have the legal authority to pass laws. That power lies with Congress, and it took Congress until 1920.

The first women's suffrage amendment was introduced in Congress in 1878. However, a suffrage amendment did not pass the House of Representatives until May 21, 1919, which was quickly followed by the Senate, on June 4, 1919. It was then submitted to the states for ratification, achieving the requisite 36 ratifications to secure adoption, and thereby go into effect, on August 18, 1920. The Nineteenth Amendment's adoption was certified on August 26, 1920.