Partially a large part of it is phenomenological (e.g. we as a field maybe haven't quite as accurately described the disease condition itself) as well as the idea that many behavioral disease states may have a complex web of neurochemical/environmental processes, operating at both short and longterm, in which a single or pattern of gene "mutations" (or differences) would not be sufficient to capture.
There is no connection between gene mutations and the massive spread of depression in the world. Depression to varying degrees of development is present in 99% of the population and the reason is not gene mutation, but some conditions that are present in 99% of the population, which lead to depression. Detailing the pathological processes in depression does not automatically lead to the cause of depression. You need to go beyond disciplinary research to understand the cause of depression. Detailed studies of pathological processes in depression will not be effective until the primary cause of depression is taken into account.