Part of the pathology of depression is not just in the connectivity and function of the ‘emotional’ parts of the brain, but also in regions responsible for the basic cognitive functions involved in reading. E.g. the prefrontal cortex (PFC) is implicated in depression because of lacking regulation of the amygdala (which at this eli5 level is responsible for mood). But, that dysfunction of the PFC is also responsible for impairments in things like decision making, thinking, or, in your question, reading.