That is a good question. The reason is that the ozone destroying reaction occurs not in the gas phase, but on the surfaces of ice crystals in the stratosphere, so called 'polar stratospheric clouds'. The stratosphere at lower latitudes does not have these clouds, and so has no surfaces on which the chlorine can absorb and enhance the reaction.
Catalysis on surfaces is as familiar as the catalytic converter in your car.
Thanks for the info.