Question

why does swift frame his proposal in terms of compassion for the poor?

Answer 1

Initially, he did that because many people in his society refers to the poor as inhumane and lack of social compassion.
He believed that barbarical tendency that are being displayed by the poor is a direct result of desperateness for them to sustain their own living and people they care about.

Answer 2

Swift explains the satirical and tragicomic solution of poverty in his Modest Proposal. He writes that poor people, namely poor Irish people under English rule could escape from this situation by selling their children as food. He gives exact statistics and ironically explains overpopulation figures, but in reality it is Swift's "serious" description of poor people's harsh times. The situation back then was so difficult for poor people that it made the author  create such a horrific and controversial art of work.