Scene 11 What is the reason for the small talk at the beginning of Scene 11?

The small talk is the awkward prelude to what everybody knows is about to happen to Blanche: it symbolizes a sense of shame and anticipation of Blanche being sent away to the mad house. Blanche's artifice has now been entirely stripped away – she is cut down to nothing. The whole cast has gathered to witness her demise, cruelly, a going-away party to which the guest of honor has not been invited.