In Act 2, what is prospero's reason for causing all but these two to sleep?

Ariel makes all but Antonio and Sebastian go to sleep, and then causes conspiratorial seriousness to settle on them as well. The situation is created as part of Prospero's project, to reinforce his idea of his brothers as villains, and act as Prospero "foresees through his art" that they will. "His project dies" if Antonio and Sebastian's deviant plot is not made; and here, Prospero again shows himself to be a manipulator of the play's events, influencing the course of the play from within. There is great dramatic irony in this situation, and in the fact that Prospero causes his brothers to do the very things that he condemns them for.