Sanskrit was a very ancient spoken language.
No language gets alphabets without being spoken first ............language is always spoken first, then aphabets are introduced to reproduce those sounds and then grammar evolves. Sanskrit grammer has evolved over the ages and underwent various modifications till it was finally perfected by Panini in 400 BC. That does not mean sanskrit did not exist before 400 BC.
Holy scriptures are written in Sanskrit because that was the language present during the time when written script became widely known. Before that the Vedas were taught and propagated through oral tradition. In fact even the knowledge of the Upanishads where part of the oral tradition and was not written down till very late. The meaning of the word Upanishad itself is 'sitting down near' ......and it is because advance students of Hinduism would sit near their Guru's and the Upanishads would be recited to them.
Most of the scriptures were written by 'Rishis' and not Brahmins. Do you understand the difference ?
Brahmins was the varna based on persons 3 gunas. Sattva, Rajas, Tamas. Those with sattva guna became brahmins. Those with Rajas became kshtriyas, Those with more Rajas and less Tamas were vaishyas and those with more Tamas and less Rajas were sudhras.
The word Raja itself comes from Rajas guna.
Now all Brahmins could be Rishis ....but not all Rishis were brahmins. Do you understand this difference ?
Maharshi Vishwamitra was a non Brahmin Rishi who is credited with the 'Gayatri Mantra' itself.
I hope you konw what that Gayatri mantra is ........most hindus seems know it...without actually knowing it.