Because in CH4 the valence electrons of Carbon are being pulled away from the nucleus with the same force (since all hydrogen has the same electronegativity).So charges are equally distributed throughout the atom and the atom is non-polar. In CH3Cl Cl has a different electronegativity than H, so charges aren't equally distributed and therefore the molecule is polar.
1) in molecule of methane (CH₄) all the atoms around the central element (carbon) are equivalent (four hydrogens) and it has a net dipole moment of 0 (vectors of dipole moments cancel each other, dipole moment is zero).
2) in molecule of chloromethane (CH₃Cl) there are different elements (one chlorine and three hydrogen atoms) surrounding the central atom (carbon) and it has higher net dipole moment.
