Why does the octet rule have exceptions?

Elements of the third raw in the periodic table and beyond can carry more than 8 electrons on their outermost shell because of the additional room (space) on their orbital d.

Only elements of the first and second raws of the periodic table can carry a maximum of 8 electrons on their outermost shell because they only have s and p orbitals available to host electrons (with the exception of hydrogen and helium they obey the duet rule).

For example, the electronic configuration of oxygen (second raw element) O: 1#s^2#2#s^2#2#p^4#
Oxygen can host 2 more electrons on its outer shell 2 and therefore 2 + 4 + 2 = 8 electrons.

However, sulfur (third raw element) S: 1#s^2#2#s^2#2#p^6#3#s^2#3#p^4#3#d^0#
Sulfur can host up to 12 more electrons on its outer shell 3, two electrons on p and 10 electrons on d and therefore 2 + 4 + 2 + 10 = 18 electrons.

This is why oxygen has an oxidation number of -2 in general with the exception of #H_2O_2# where the oxidation number is -1; however, sulfur has many oxidation numbers such as -2 in #S^(2-)#, +6 in #SO_4^(2-)# and +4 in #S_2O_3^(2-)#.

If we look at the Lewis structure of #SO_4^(2-)# we can count 12 electrons around sulfur: