Friends:

Why do some gain Nibbãna, while most do not?

Sakka, the king of the 33 Gods, once asked the Blessed Buddha:
Venerable Sir, what is the cause and reason, why some beings here do not
attain Nibbāna in this very life? And what is the cause and reason, why
some beings here do indeed attain Nibbāna in this very life?
Knowing this being's addiction to divine sex delight the Buddha answered:
There are, King of the thirty-three Devas, forms experienceable by the eye,
sounds experienceable by the ear, smells experienceable by the nose, touches
experienceable by the body, and mental phenomena experienceable by the
mind, that all are attractive, charming, agreeable, pleasing, tempting, alluring,
and tantalizing. If a Bhikkhu hunts for delight in them, welcomes them, and
remains holding on to them, his mind becomes addicted to them, and clings
to them. Any being mentally dominated by clinging cannot attain Nibbāna!
This is the cause and reason, King of the Devas, why many and most beings here
do not attain Nibbāna in this very life... Again: There are, King of the Gods,
forms experienceable by the eye, sounds experienceable by the ear, smells
experienceable by the nose, touches experienceable by the body, and mental
phenomena experienceable by the mind, that all are attractive, charming,
agreeable, pleasing, enticing, tempting and tantalizing.  If a wise Bhikkhu
avoids seeking any delight in them, does not welcome them, and does not
remain holding on to them, then his mind doesn't become addicted to them,
Any being mentally wholly freed from clinging can indeed attain Nibbāna! 
This is the cause and reason, King of the Devas, why some beings here can
attain Nibbāna in this very life, while most neither can, nor even want that....



king Sakka and a few of his hundreds of dear nymphs...

More on the Deva King Sakka (Indra) of the 33 Gods (Tāvatimsa):
https://What-Buddha-Said.net/library/DPPN/sa/sakka.htm
https://What-Buddha-Said.net/library/DPPN/t/taavatimsa.htm
http://en.wikipedia.org/wiki/Indra

Source:
The Grouped Sayings by the Buddha. Samyutta Nikāya. Book IV 102
The 6 senses section 35. Thread on Sakka's Question: 118.
http://store.pariyatti.org/Connected-Discourses-of-the-Buddha_p_1379.html
https://What-Buddha-Said.net/library/ati_website/html/tipitaka/sn/index.html

Why and Why not?