Why do people assimilate?

Observe those around them; take on their mannerisms in dress and speech, sometimes even in belief system - even if only on the surface. They embrace the culture of their new surroundings - to fit in. Learn the language.

They do this to become part of the group that they have chosen to join, otherwise they would not have made the move. People, as a group want to be accepted by the group / pack. They want to fit in.