The Seneca Falls Convention met for the purpose of pursuing women's rights. Women in the United States at that time had very few rights. One right they did not have was the right to vote. The Declaration called for that right, among others.
The issue of women's rights was coming to the fore at this time because of changes that were sweeping America. There were many efforts to improve social conditions of all sorts. One of these was the abolitionist movement. Many women were involved in that movement. Their involvement in fighting for others' rights led them to want to fight for their own rights. This is why they met in Seneca Falls and it is why they wrote the Seneca Falls Declaration.