Because Nucleus is composed of protons and neutrons..
in the nucleus of any atom,the no. of protons and the no. of neutrons are same.Also in the nucleus the protons and the neutrons are intermixed not remain seperated.In nucleus two protons never touch each other because between any two proton there is always a neutron which contradicts the force of repulsion.
BECAUSE THE REPULSIVE FORCE OF PROTONS IS LESS THAN THE ATRACTIVE FORCE BETWEEN NUCLEONS.