Why was the Corwin Amendment in 1861 not ratified?

The Corwin Amendment was passed by Congress, however it was only ratified by three states. Since it was passed for ratification without a time limit, if 35 more states were to ratify it, the Amendment could still become law. That's unlikely, however.

The amendment (passed by a lame duck Congress in 1861) would have prohibited non-slave states from proposing amendments to eliminate slavery, which in effect the 13th Amendment did. The application of the amendment ex post facto would present legal questions that would likely prove unsolvable.