I was not aware of the Corwin Amendment so firstly thanks for bringing it to my attention.
It seems like an absolutely insane thing for the Republicans as an abolitionist party to pursue but there you go.
The Republicans were
not an abolitionist party. Abolitionists were nearly all Republicans, but most Republicans were "Free-Soil" men - opposed to the expansion of slavery, but not interested in liberating the oppressed negro. Many of them opposed slavery because it brought blacks into the US and into new parts of the US. Abolition would set blacks free to move to all parts of the country, which they found disgusting. An 1860 Republican campaign pamphlet mocked the Democrats for claiming Republicans favored "nigger equality". The pamphlet asserted that the Democrats wanted to expand slavery into the Territories, whereas the Republicans would reserve the Territories for free white men - and "where there are no niggers, there can be no nigger equality." This was a
Republican,
pro-Lincoln pamphlet.
Furthermore, as much as they might dislike the presence of blacks in America, they recognized that those here already had to live somewhere, that they lived in the South, and that slavery was the institution which kept the blacks in the South under control, besides being an enormous capital asset of Southerners. Thus they understood that Southerners were very upset by abolitionist proposals, and by John Brown's attempt to foment slave insurrections. Secession was wrong, but abolitionist threats helped cause the panic that led to it.
Clearly they felt that Secession and Civil War were less preferable to ongoing slavery, and in a way I can understand that.
Only a few abolitionists were ready to fight a war to liberate slaves
now. or to risk the break-up of the country over it. (Prominent abolitionist Wendell Phillips spoke in favor of recognizing secession, because it would purge the US of slavery. He was booed off the platform.) Nearly everyone feared war far more than than they cared about slavery.
The great majority of Americans who disliked slavery hoped for a peaceful, gradual end to the institution. Lincoln, in the debates with Douglas, thought might take a hundred years for the final extinction of slavery (i.e., there might still be a few elderly slaves then).
I have trouble seeing it getting 3/4 of states to ratify but it was passed by Congress so maybe.
Another point is that there was no Constitutional authority for the Federal government to free slaves in a state. Thus Lincoln said that as the proposed amendment would only affirm existing conditions, it made no difference.
There is, incidentally, some question as to whether the amendment would have any effect at all. Its effect was supposed to be that no future amendment could touch slavery; but a legislature may not bind the hands of its successors, by passing a law which is irrevocable. Anything a legislature may do, a future legislature may undo. The same principle would apply to Constitutional amendments.