Why do different isotopes of an element generally have the same physical and chemical properties?

The correct answer is because they have same number of protons but different number of neutrons.

Explanation:

Isotopes are atoms of the same element but differ only in the number of neutrons in the nucleus, i. e. they have same atomic number but different mass number.

Mass number is affected as they have different number of neutrons, thus effecting their physical properties.

The number of electrons and protons are same, i. e. their atomic number is same and thus their chemical properties are same as chemical properties are determined by the atom's electronic configuration and that relates to number of protons.