The difference is size: κωμόπολη = (small) town (http://www.wordreference.com/gren/%CE%BA%CF%89%CE%BC%CF%8C%CF%80%CE%BF%CE%BB%CE%B7), while πόλη = town, city (http://www.wordreference.com/gren/%CF%80%CE%BF%CE%BB%CE%B7).
Why "city" is not accepted ? I am not English so I apologize if the answer is obvious to native English speakers.
City is bigger than a town so to show the difference in Greek we use κωμόπολη=town and πόλη=city. Πόλη can be used as "town" but κωμόπολη can't be used as "city".