“They say to him, ‘Because no man has hired us.' He says to them, ‘You also go into the vineyard.' ”
So they inform the landowner that the reason that they are still there, (having stood there be it noted through the heat of the day), is because no one has hired them. We can imagine how they were feeling, and even more their great delight when the landowner hires them at a time when they were past hope. Their pay for work at the end of the day might be small, but it will be better than nothing, and they are grateful. It may at least buy some stale barley bread for their families to feed on.