When a charged coulomb is brought near small piece of paper it attracts the piece why?

This is because when charged comb is brought near the bits of paper, separation of charges inside the bits occur. Comb possess negative charge on it and thus attract positive charge of paper towards it and repel negative charge on the other end. Now as distance between positive charge of paper and negative charge of comb is less than distance between negative charge of comb and negative charge of paper, so according to coulomb's law, force will be more between positive and negative charge. So comb will attract the bits.