Why would seasonal affective disorder be more severe in the northern hemisphere?

No reason: I can think of that it would be different in the northern than the southern hemisphere. The critical factor is thought to be a decrease in daylight during winter months. That would be the same the farther one is from the equater in either direction.

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.