Final answer:
The 15th Amendment was seen as insufficient because it allowed states to impose discriminatory voting qualifications that excluded African Americans, and it did not extend voting rights to women, thus failing to fully ensure equal universal suffrage.
Step-by-step explanation:
Some people felt the 15th Amendment did not do enough because, while it granted black males the right to vote, it didn't address other discriminatory practices that effectively disenfranchised them. For example, states used seemingly race-neutral qualifications like literacy tests and land ownership to restrict voting rights. These tactics persisted for decades, alongside violence and intimidation, such as lynching, which hindered the African American community's ability to safely and effectively exercise their voting rights.
Furthermore, the 15th Amendment did not address women's suffrage at all, which was a major disappointment to women's rights activists who had also been part of the abolitionist movement. The amendment's language specifically excluded women, further perpetuating gender inequality in voting rights.
Overall, the systematic discrimination, along with the lack of inclusive voting rights for women, made the 15th Amendment seem insufficient in the eyes of many who were fighting for equality.