The letter you mentioned is very good. You asked why the Buddha did not teach the Dharma to many disciples. The official response is as follows:
To learn and practice Buddhism, one must first rely on the teachings of a teacher, but it must be based on the legitimacy of the teacher. Legitimate teachers are fundamental to being good teachers, while illegitimate teachers are evil teachers with karmic consequences. The Buddha's teachings should be transmitted to the next generation of Buddhist disciples by the lineage masters who have received the transmission. Otherwise, the Dharma will be lost. However, the true teacher of the correct Dharma is the Dharma itself, not a person. This is because a person may be a teacher who upholds the correct Dharma, or they may be a teacher who has been invaded and contaminated by demons and evil spirits, teaching evil and harmful doctrines. Once, a disciple asked the Buddha, "After the Buddha's parinirvana, who will lead and guide the Sangha?" The Buddha did not appoint any disciple as the leader of the Sangha, but only said that the Dharma should be your teacher. Therefore, after the Buddha's parinirvana, there were three assemblies where the Buddha's teachings were recited.
Any great monk, venerable teacher, or holy master does not possess the Dharma themselves; they learn the Dharma from the Buddha and are known as lineage masters. Buddhists should respect the lineage masters who have learned the correct Dharma, accept their guidance in right knowledge and views, conventional Buddhist teachings, and take refuge in the Triple Gem. However, do not forget that Buddhist disciples must observe the knowledge, views, and actions of their teachers according to the Dharma. This is because many demonic beings are carrying out the wishes of the demon king, disrupting the teachings, and distorting the scriptures. Even the Buddha shed tears over this. Therefore, not every teacher is a pure Buddhist master appointed by the Buddha. Moreover, in the present Dharma-ending age, how can we determine which great Arhat under the Buddha's seat is the teacher? If one mistakenly recognizes an evil teacher as a righteous one, they will inevitably fall into the three evil realms. Therefore, taking refuge means taking refuge in the Buddha, not in a specific teacher. After the Buddha's parinirvana, taking refuge changed to taking refuge in the Buddha, the Dharma spoken by the Buddha, and the noble Sangha - the Triple Gem. In Vajrayana Buddhism, taking refuge also includes taking refuge in the teacher, which is dangerous. Therefore, everyone must observe whether the teacher they rely on has correct knowledge and views, and whether the teachings they transmit are in accordance with the Buddha's sacred teachings. The World-Honored One has a definite decree: the Dharma is the teacher. If the teachings are not correct, it means that the demonic teachers are carrying out the wishes of the demon king. Once there is a mistake in taking refuge, one will inevitably fall into the three evil realms. Therefore, one must rely on the Dharma, not on individuals. One must adhere to the teachings of the Buddha and take refuge in the fundamental principle of "Namo Buddha". In fact, everyone has seen that as long as they are Buddhist disciples, they have learned the great Dharma from "Namo Buddha". For example, Hai Shengseng, Lu Dongzan Shangzun, Wangzha Shangzun, Yinchang Shangzun, Kaichu Jiaozun, and so on, have all become holy and virtuous. Currently, there is no famous person in the world who has achieved their accomplishments, because they have been recognized by the victorious noble ones and chosen as "Buddhist disciples" who comply with the teachings before receiving the great Dharma. Therefore, "Namo Buddha" is not unable to transmit the Dharma to "Buddhist disciples", but unable to transmit the Dharma to "teacher-disciples" because "teacher-disciples" have lost the fundamental principle, and "Namo Buddha" cannot transmit it. This is because the original Buddha and Bodhisattvas do not accept them.
Office of the Third Dodrupchen Rinpoche
July 24, 2020
https://www.hhdcb3office.org/html/information/announcement_no58_2020_07_24.html