Because after the discovery of Isotopes-
The discovery of isotopes indicated that all the atoms of the same element are not perfectly identical in all aspects.
Atoms of the same element having different masses are called as isotopes. eg. 1H1, 1H2, 1H3
After the discovery of isobars it was observed that-
Atoms of different elements are not different in all respects; atoms of different elements having same atomic mass are called as isobar. For example argon and calcium atoms have same atomic mass of 40.