Well, social mobility is the change in social status relative to others, which can correlate with income, yes. It also correlates with things like education and skills, type of occupation, etc.

> Why are they being perevented ?

Well, education is expensive -- and has an opportunity cost -- so low-income people often cannot afford to improve their skills and earn more income. If I am poor, and I have enough intelligence to become a good scientist, engineer, or artist, but cannot afford to go to university and gather the requisite skills, then this is a loss to society.