The declaration penned by Olympe de Gouges was famously ineffective. Why do you think that was the case?

I do not think that there is much mystery here.  De Gouge's declaration was ineffective because it was way before its time.  Because of this, it had no chance to really have much of an impact.

There is a similar well known case in the United States.  Abigail Adams wrote to her husband, John Adams (who became the second president of the US) asking him and the others who were writing the Declaration of Independence to "remember the ladies."  She was asking for more rights for women.  Her husband, who usually treated her respectfully, said in response "I cannot but laugh..."

The idea that women could be equal to men was laughable at this time.  It would have been like declaring the equality of black people in the South in 1820 or like arguing that animals deserve the exact same rights as humans today.

When your ideas are before their time, they are going to be ineffective in the short run.  That is what happened to de Gouges.