If Aristotle has a concern for virtue, justice, and the good, why does he spend so much timetalkin about the"middling regime"?

In book 4 of the politics, Aristotle refines his classification of regimes and discusses "the middling regime,which he seems to identify as " best for most cities an most human beings". Then he go's on to sayhow to avoid revolution in various regimes, even offering the tyrant advice as to how to avoid being overthrown.