What would be a motivating reason why abolitionists opposes annexation of new western territory during the early 1800s?

Maybe " If  they seize this territory, to what effect of the people we 'fight' for will this go into reason?",then,"They might make it a market, or even a center of slavery! We need to stop this." or at least in the words of a speech.

i think the fact that abraham lincoln was a tyrant

or probably like what the other person said from sitting in the chair to get assasinated xd

~batmans wife dun dun