Because ratifying such an amendment would require 46 states, assuming all 15 slave states voted against it. Please do the math and then let me know if that is satisfactory enough.
A nation that has the constitutional ability to peacefully abolish slavery - and refuses to use it - is no better off than a nation that doesnt have the constitutional ability.
That ability did not exist in the U.S. in 1860.
“Please do the math and then let me know if that is satisfactory enough.”
It is not.