due to the energy gap between the subshells.
students ask me this, or similar questions, all the time in class. Students often thinking of the "half full, totally full" rules as being set in concrete, but they're not.
to promote an electron from any given sub-shell up to a higher energy sub-shell requires energy. Sub-shells are indeed more stable if they are totally full or half full.
So, the promotion of an electron to a higher energy level (sub-shell) is only ever favourable if the energy gain from the extra stability is greater than the energy required from the promotion.
In this case the "gap" between 3s and 3p is too big, so Cl stays as [Ne] 3s2 3p5. For other elements, the "gap" is small enough to allow for promotion of an electron (for instance Cu).
see also the image of the quantum sub-shell splitting
(this site has a decent image )