We may never know the purpose of the founding Republican Party members. Their founding party platform said the purpose was to stop the expansion of slavery. But there may have been a lot of smokey, backroom deals that could not stand the light of day.
My guess, the purpose of the party at its founding was to support the economic and political best self-interest of the founders.
For the purpose of this post, let's stipulate you are correct: the Republican Party was formed to abolish slavery.
I marvel that just a few short years later members of the Republican Party - with the first Republican Party president's tacit approval - were voting to adopt the Corwin amendment which would have enshrined slavery into the United States Constitution forever.
It doesn't add up.
It wouldn't have enshrined slavery into the Constitution, it would have enshrined the right of each state to choose how to handle slavery. Practically speaking it would have ensured slavery in the short-term, but anti-slavery elements (particularly those who considered the eventual abolition of slavery to be Providential) could have held out hope that the individual slave states would eventually choose abolition.
Furthermore, the perceived choice at the time was between Southern secession and a compromise like the Corwin Amendment. The Corwin Amendment would not have freed any slaves, but neither would have secession. So it might be understandable why those against slavery might not have a strong opinion on the matter.