Why is seasonal affective disorder more severe in the north?

More prevalent. Perhaps you mean that seasonal affective disorder is more prevalent in the northern hemispheres -- not really more severe. An individual case might be similar in severity, no matter where the person suffering it lives. But the condition is more frequently seen in northern latitudes -- where in fall-winter, daytime light exposure is less than it is in southern latitudes.
SAD. Sad is more prevalent in regions with colder darker wonters allowing for less interaction with other humans and the environment.