So far this is what i have learned so far. The varna(class) is a  Hindu term for caste, a social division into which a person is
born. There are four major castes in Hindu society: Brahmin, kshatriya,
vaishya, and shudra. The first three castes are considered
“twice-born.” Each Varna can be divided into a number of jatis. Below
these four varnas are the untouchables or simply dalits.

Today was a Mahashivaratri and i believed that many devotees are observing this rite in Nepal and India.

But one question why are dalits and low caste people not allowed to enter the temple?

Is it because of the laws of Manu?

Now again this social structure is not permanent. I believed a Brahmin can felled into shurda if he does not conform with the religious rite. For example the Kiratas who are recognized as Kshatriyas are degraded into lower status for not following some of the rites. The low caste are also promoted to higher level through gaining merits.

Now what are these rites that one have to follow in order to maintain high caste. Does anyone have any ideas?
I understood that Kirat society do not have varna system and did not used to practice even in vedic times. Is this one of the reason why the Kiratas became degraded Kshatriya.(Manu's Dharmashastra (X.44)
Another thing could be that Kiratas did not engage in sati and dowry system either hence they were seen as not conforming to vedic rules and norms, hence they were degraded.
In short, Kiratas were unorthodox Hindus.
But here's the question. Do you thing that having not practice varna-sati-dowry system was way better? The varna was the one that have really destroy the Nepali society. The dowry system on the other hand was also a evil culture.
I discussed this with my lecturer and she was flabbergasted.

Hope to see some inputs.
thanks