To go full-on Machiavellian for a second: The middle class has enough material wealth to have "something to lose", but not enough material wealth to individual

To go full-on Machiavellian for a second: The middle class has enough material wealth to have "something to lose", but not enough material wealth to individually upset the status quo too much. On top of that, there are enough resources to go around to allow a large group of people have that same approximate level of wealth. Ergo, the middle class is a tremendous stabilizing force in a society, and they're "guillotine insurance" for the rich.