Now this is what i found out. The Vishnu Purana(an ancient Hindu texts that was written around 3rd century BC) mentioned that the chatur-varna or four class social system was absent among the Kiratas in Kiratdesa in the east and the Yavanas(Greek) and Kambojas(Iranian), etc. in the pashchima or west. (Vishnu Purana, 2.37).

This tells us that Nepal did not used to have the caste system.
So what was the reason behind the existence of caste system one may ask?

After studying some early history of Nepal, i have to say that the caste system arrived in Nepal with the arrival of Lichchhavi from northern India who overthrew the Kirat dynasty. The Lichchhavi promoted the Hindu religion by constructing many temples. They adoptedSanskrit as the official language and promoted sculpture system. When the Mallas superseded Lichchhavi dynasty, they made every attempt to promote Hinduism. With the help of Brahman priests, the Malla rulers introduced Varna system and stratified people on the basis of caste structure. Theyassumed that enforcement of caste hierarchy would promote social cum political stability. They expected lower caste group and non-Hindu communities gradually adopt the rituals and ideology of high caste Hindu. Furthermore, Jayasthiti Malla (1360-95) started promoting social reform activities on the threshold of Hindu doctrines. He invited 5 Brahmins from India by requesting them to make the rules as they wished. There after, he formulated a religious code named Manav Naya Shastra in 1380 AD, which is the oldest law of the world, where various discriminative laws had been included.

what do you thins folks? i'm gonna discuss this with my lecturer.