Because Kṛṣṇa, or God, is pure. Arjuna said, paraṁ brahma paraṁ brahma pavitraṁ paramaṁ bhavān: "My Lord Kṛṣṇa, You are the supreme pure." So unless we are pure we cannot approach Kṛṣṇa. That is the statement in the śāstra. Without becoming fire, you cannot enter into fire. Similarly, without becoming completely pure, you cannot enter into the kingdom of God. That is accepted by all religious system. The Christian system is also like that, that without becoming pure you cannot enter into the kingdom of God.
So the human life is meant for this purification. We are working very hard for getting our daily bread. People are not getting their bread sitting idly. That is not possible. They are working very hard. This nice city of Denver is there. It has not sprung up from the jungle or desert. One had to work very hard to make this city so nicely, perfectly standing. So we have to work. If we want happiness, then we have to work. There is no doubt about it. But Kṛṣṇa says that yānti deva-vratā devān (BG 9.25). Somebody is working to become happy within this material atmosphere by becoming very big man within this world, or a little more intelligent, they are not happy in this life, but they want to become happy in the next life. Sometimes they go to the higher planetary system. So yānti deva-vratā devān pitṛn yānti pitṛ-vratāḥ (BG 9.25). So as you work, you get the desired result. But at the last line, Kṛṣṇa says, mad-yājino'pi yānti mām: "If you work or if you worship Me, then you come to Me." Then where is the difference between going to Kṛṣṇa and keeping within this material world? The difference is ābrahma-bhuvanāl lokāḥ punar āvartino 'rjuna (BG 8.16).