Why were the Macedonian believers poor?
The Macedonian believers were poor because Macedonia was a rough, mountainous region of northern Greece. Its people largely eked out a living agriculturally, and such terrain produced little material wealth. The context states plainly that they had none and were poor themselves.
Les explains that Paul is writing about the churches of Macedonia, which was northern Greece, near Yugoslavia, while Achaia was southern Greece around Athens and Corinth. He notes that all of Greece is mountainous. In that rough, mountainous countryside, where most people eked out a living agriculturally, the believers had no material wealth—they were poor themselves. Paul refers to these congregations in that mountainous area of Greece.
Moreover, brethren, we do you to wit of the grace of God bestowed on the churches of Macedonia;