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.
I think that I am stuck a bit on the question. The line that seems to be trapping me a bit was "famously ineffective." I guess I am having difficulty seeing why de Gouges was perceived in such a light. Certainly, one can argue that she came short in achieving her goal of equal rights for women, as outlined in her declaration. Yet, I think one would have to assess whether or not it would be fair to pin this squarely on her shoulders. She is writing and asserting her beliefs in a time of great political upheaval in France. Many were "famously ineffective" while trying to speak out during the Reign of Terror. The guillotine had a tendency of doing this to people, and the mob rule of the time period rendered most as "ineffective." While she might received criticism for her failure to cohesively organize a feminist force in France at the time, I think she should get much credit for even attempting to do so in the political climate of the post- Revolution euphoria, where fear and intimidation dominated the landscape. Her condemnation of Robespierre, one of the reasons she ended up getting killed, also demonstrated a great deal of vision and a keen understanding of the political situation.