Because isotopes of the same element only differ by a few neutrons. An example is carbon-12 and carbon-14 isotopes which differ by 2 neutrons in their atomic nuclei.
Explanation:
Isotopes of the same element share the same chemical properties because they bear the same number of electrons (remember electrons are what are involved in chemical reactions). Therefore one can't use chemical methods to separate isotopes.
Separating isotopes require mechanical separation using ultra-high-speed centrifuges with very high-resolution power such that the little mass difference between the isotopes (due to differences in neutron numbers) can be used to separate them.
