Genesis 29:8

‘And they said, “We cannot until all the flocks are gathered together, and they roll the stone from the well's mouth. Then we water the sheep.” '

The answer was that it was because the stone could not be moved. This may have been because there were not enough men there to move the stone. Most of the keepers of the sheep were probably women. Alternately it may have been because it was part of the agreement in respect of the private well that the stone not be removed until all were present. But we are probably intended to get the idea of the diligence of Jacob compared with the dilatoriness of the shepherds.