That special mercy of the power to preach is given to a devotee who does not want anything material from his spiritual master but wants only to serve him. The story of the demon Ravana illustrates this point
SB Canto 5
O supreme unconquerable Lord, when they become absorbed in thoughts of material enjoyment, Lord Brahmā and Lord Śiva, as well as other demigods and demons, undergo severe penances and austerities to receive my benedictions. But I do not favor anyone, however great he may be; unless he is always engaged in the service of Your lotus feet. Because I always keep You within my heart, I cannot favor anyone but a devotee.
In this verse the goddess of fortune, Lakṣmīdevī, clearly states that she does not bestow her favor on any materialistic person. Although sometimes a materialist becomes very opulent in the eyes of another materialist, such opulence is bestowed upon him by the goddess Durgādevī, a material expansion of the goddess of fortune, not by Lakṣmīdevī herself. Those who desire material wealth worship Durgādevī with the following mantra: dhanaṁ dehi rūpaṁ dehi rupavati bharyam dehi. "O worshipable mother Durgādevī, please give me wealth, strength, fame, a good wife and so on." By pleasing goddess Durgā one can obtain such benefits, but since they are temporary, they result only in māyā-sukha (illusory happiness). As stated by Prahlāda Mahārāja, māyā-sukhāya bharam udvahato vimūḍhān: (SB 7.9.43) those who work very hard for material benefits are vimūḍhas, foolish rascals, because such happiness will not endure. On the other hand, devotees like Prahlāda and Dhruva Mahārāja achieved extraordinary material opulences, but such opulences were not māyā-sukha. When a devotee acquires unparalleled opulences, they are the direct gifts of the goddess of fortune, who resides in the heart of Nārāyaṇa.
The material opulences a person obtains by offering prayers to the goddess Durgā are temporary. As described in Bhagavad-gītā (7.23), antavat tu phalaṁ teṣāṁ tad bhavaty alpa-medhasām: men of meager intelligence desire temporary happiness. We have actually seen that one of the disciples of Bhaktisiddhānta Sarasvatī Ṭhākura wanted to enjoy the property of his spiritual master, and the spiritual master, being merciful toward him, gave him the temporary property, but not the power to preach the cult of Caitanya Mahāprabhu all over the world. That special mercy of the power to preach is given to a devotee who does not want anything material from his spiritual master but wants only to serve him. The story of the demon Rāvaṇa illustrates this point. Although Rāvaṇa tried to abduct the goddess of fortune Sītādevī from the custody of Lord Rāmacandra, he could not possibly do so. The Sītādevī he forcibly took with him was not the original Sītādevī, but an expansion of māyā, or Durgādevī. As a result, instead of winning the favor of the real goddess of fortune, Rāvaṇa and his whole family were vanquished by the power of Durgādevī (sṛṣṭi-sthiti-pralaya-sādhana-śaktir ekā (Bs. 5.44)).