There is no record of John Quincy Adams proposing such a Constitutional Amendment. It is possible that this statement is a misquote or a misattribution.
However, the issue of slavery was a highly contentious issue during the mid-19th century, leading up to the American Civil War. Many politicians and activists, including Adams himself, worked to abolish slavery and promote civil rights for African Americans.
One of the key factors that prevented the passage of such an amendment was the political power and influence of Southern slaveholders, who were vehemently opposed to any efforts to limit or abolish slavery. The Southern states were concerned that any restrictions on slavery would threaten their economic and social systems, and they were willing to use their political influence to block such efforts in Congress.
Furthermore, the process of amending the Constitution is intentionally difficult, requiring approval by two-thirds of both the House of Representatives and the Senate, as well as ratification by three-fourths of the states. This high threshold means that even popular or well-intentioned amendments can fail to pass due to political opposition or logistical challenges.