Because in mendlef experiment it is said that  periodic property depends on atomic wt.. nt atomic no,,,mendeleef done well in his calculation...he is almost right...placing all elements in proper place..but some eroor occurs when time passes....isotopes come in the sense,,,having same atomic number bt diff,, mass mo...then how mendleef place on his table.....this problem leads to the occurence of new theory...neil bohr,,,that periodic property is the function of their atomic number...because atomic number represents number of electron....and in chemistry every property like every reaction, chemical and physical change depends upon change in its atomic number....i.e change in its electron....