Because you cannot develop devotional service without pious life. Yeṣām anta-gataṁ pāpam. If you want to become associate with Kṛṣṇa, then you cannot act as a sinful man. Just like a criminal is not allowed to come out of the jail. He has no freedom. Similarly, if we act sinfully in this life, then we'll have to remain within this material world, one body after another. So we, we have to give up sinful activities. Therefore we forbid our students, no illicit sex, no meat-eating, no gambling, no intoxication. Because these are the pillars of sinful life. So we have to give up these things to accelerate our promotion to devotional service. We cannot go on doing this and that at the same time. It is something like that, you ignite fire and pour water. It will be useless attempt. If you want to burn the fire blazing, don't put water on it. Keep it dry. Similarly, if you want to advance in Kṛṣṇa consciousness, then you cannot indulge in sinful activities. And when you are proved that you are no more sinful, yeṣām anta-gataṁ pāpam, one who has become freed from the reaction of sinful life, yeṣām anta-gataṁ pāpaṁ janānāṁ puṇya-karmāṇi...
You have to do something. So if you engage yourself twenty-four hours in pious activities, pious activities in goodness, that is also tainted. But when you actually engage yourself in the devotional service of Vāsudeva, as we were discussing this morning, the verse, vāsudeve bhagavati bhakti-yogaḥ prayojitaḥ, janayaty āśu vairāgyam... (SB 1.2.7). So even if we are accustomed to sinful activities on account of bad habits, still, we can engage ourself in devotional service of Vāsudeva. So those who are sincerely devotee of Vāsudeva, sometimes due to their past habits, may commit something sinful, not knowingly, but habituated; that is excused by Kṛṣṇa. Api cet su-durācāro bhajate mām ananya-bhāk.