One of the major differences between tragedy and comedy for Aristotle was the nature of their protagonists. Tragic protagonists were people better than average and comic protagonists were people worse than average. For Aristotle, the conception of nobility of character was typically linked to being born to a noble family. This was in part connected with the notion that a tragic plot had to have a certain scope and seriousness.
The lives of kings or nobles had a major effect on the city as a whole and thus were more serious in consequence than the private lives of ordinary citizens. The decisions of Creon, Oedipus, or Agamemnon determine the future of city states rather than just affecting themselves and their families.
Finally, only the downfall of a noble and great character will arouse fear and pity. The downfall of a petty or villainous character will not evoke those uniquely tragic emotions.