I'm not going to try and answer this question in great detail because the historical details are not part of what I really understand, but I can tell you this.
Lincoln often used compromise in both his political and his legal dealings with the world. Though he knew the compromise would never be accepted, he also knew that the government of the north could never, should never be forced to accept the conditions the south was proposing. In other words, what he was arguing was that if the south won this round, they might as well have won the election. They could keep on making demands whenever it suited them. ad libitum. No!! Lincoln was saying!! No!
We must deny them this demand. We must not give in. The only compromise he was willing to consider was that the dividing line not be allowed to expand further west. That new territories not be governed by slaves or no slaves. That these territories would simply not part of the union. Think about that for a moment. It meant that everything west of the Mississippi and South of the Mason Dixon extended would not be admitted to the union. Places like Arizona, Nevada, Southern California would not exist the way we understand them.
The answer is bold and concerned.