Why did Paul take up collections for the poor Christians in Jerusalem?
Paul took up collections for the poor Christians in Jerusalem because those believers had sold their property and cash, expecting the Kingdom to come soon. When the Kingdom did not immediately appear, their funds ran dry, leaving them destitute. Paul therefore gathered aid from his Gentile congregations to relieve their poverty.
Les explains that the early Jerusalem believers were so willing to sell their property and lay the proceeds at the apostles' feet because they expected the King to set up the Kingdom shortly. In the Kingdom, there would be no poverty, and everyone would have everything. According to the Old Testament program, they believed the Messiah's rejection would be followed by wrath and then the King. With that prospect, they did not worry about keeping houses and lands, and they gave gladly so the apostles could preach the Kingdom and more Jews would believe, hastening the King's coming. But, as Les points out, it all came to nothing. When the Kingdom did not come as they expected, their funds ran dry. This is why Paul, whenever he went among his Gentile congregations, was always taking up collections for the poor at Jerusalem; those believers had cashed everything in and were left destitute.