A psychological disorder is a repeated pattern of behavioral or different psychological symptoms that causes stress and impacts different and numerous parts of an individuals life. Chronic procrastination does cause a lot of stress and is a repeated habit that people do all the time. More so, chronic procrastination is both mental and physical, not just a person being lazy. Often times people who suffer from it want to do the work but fear and guilt hold them back which only creates more of the fear and guilt. Also like many other psychological disorders, chronic procrastination can be treated and slowly fixed to a livable life. Why would it not be a psychological disorder?
And even so it may be a symptom of a disorder rather than the disorder itself.
But in many cases the person just needs to stop procrastinating. OR if the procrastination is mild enough and the person is aware of their tendency to procrastinate and can compensate for it and function then it's not a disorder. Something is not a disorder if there is no impairment in function.
Procrastination is just waiting until the last minute to do work. If you put off working for different reasons, such as another disorder or something along those lines, then that is another story. But classifying "chronic procrastination" as a psychological disorder just gives people an excuse to let their procrastination get out of hand. It is possible to force yourself to complete your work, laziness is the main contributor to procrastination and that can be overcome with a little bit of effort.