There used to be two kind of weapons in ancient times:
1) Astra - Weapons powered by Mantra
2) Shastra - Physical weapons
One can obtain astra only after pleasing the corresponding devata.
For ex: brahmastra by pleasing brahma. Pashupatastra by pleasing shiva etc.
For many Astras, corresponding devatas used to have their Amsha in the body of person who obtain the astra. So in order to use astra one needed to have these thing
- Any physical object like bows, wood , blade of grass etc
- Corresponding devata (called astra devatas) in the body
- Mantra for the astra.
Suppose if devata leaves the body of the person , then even if knows the mantra, then it would be of no use . It happened to Arjuna , after krishna left physical world. He knew all mantras , but astra devatas started leaving his body. He left helpless unable to do anything.
Each Astras used to have its own peculiar properties.
For ex: Brahmastra could destroy any of the creation of brahma.
In last yugas(Dwapara,treta,satya) , people used to have much better memory powers and physical strengths. They used to memorize everything and there wasn't much need for writing down any of these scriptures.
Astra Vidyas used to be taught in strict disciplistic succession and only right person used to be given this vidya to prevent misuse. It was never written down, when Kali yuga came.
Also there is something called Yuga dharma. Just like we have basic law of physics, things used to work much differently in previous yugas. Kali is the time , where materialism dominates. So these astras may not be suitable for this yuga.