The answer to 'why is my sewer line slow draining' is that the sewer line is likely clogged or obstructed. Common causes include buildup of debris, tree roots growing into the pipe, or a broken/collapsed section of the sewer line. Symptoms include slow or gurgling drains, sewage backups, and unpleasant odors.
In many cases, a slow draining sewer line can be cleared with a plunger, drain snake, or high-pressure water jetter. However, if the problem persists or the sewer line is severely damaged, you'll likely need to call a professional plumber to inspect the line and make any necessary repairs.