How did the knowledge of great weapons like Bramhastra got lost in Hinduism tradition? As can be seen in Ramayan and Mahabharat, Gurus used to give knowledge of weapons to their shishya which were then used to protect Dharma. But eventually this knowledge was lost and Bharat couldn't fight invaders. How did this happen? Why weren't Devas invoked through yajnas when Sanatan Dharma followers were ethnically cleansed at the rate of millions by foreign invaders? At time of Mahabharat Indra himself helped Arjuna by granting him weapons. What happened now?
Normally, a competent Guru is required to understand a subject at deeper levels. It applies to every branch of knowledge, be it SPIRITUALITY, or science or arts or something else.
Again, a competent Guru will not share all his knowledge to every passerby. He/She should find a suitable disciple, who is competent enough to receive the knowledge.
Sri Ramakrishna Paramahamsa used cry for worthy disciple, while Narendranath (Swami Vivekananda) was searching for a worthy Guru.
It happens in every branch of science. If one finds the divine weapons mentioned in the Epics like Ramayana or Puranas, but unable to find them in the present age, that situation can indicate innumerable reasons.
For example Sage Viswamitra did not impart his knowledge in divine weapons for every king of his times. He waited for long for the birth of Sri Rama.
A worthy Guru waits for a worthy disciple. That is the underlying principle.
Drona did not impart all his knowledge in archery to every one. He imparted that knowledge to the Arjuna only. Out of love towards his son Aswathama, he taught the latter the knowledge in Brahmasirsha, which is all powerful, which proved to be detrimental to the mankind.
Competency of disciple includes his/her balance of mind in utilising the knowledge that acquired and searching for a worthy disciple in turn for passing on the knowledge.
With the elimination of most of the kings in Mahabharata, the society degraded and humans lost the ability to acquire and retain knowledge of great things.
That was how we lost touch with the pure SPIRITUAL concepts of Vedas and got degraded into ritualistic society.
Similar was the case with other branches of knowledge, knowledge of divine weapons being one among them.