In simple terms, Chara is not in the mood to talk right now. It's 6:30 AM, and they want to walk to school without having to work more than they have to. They already know that they have to tolerate being at the same school as Henry, who is not only a human (which Chara already hates), but Henry is also an avid monster hater. Even worse.

Neither sans, Toriel, nor Papyrus are available to walk Chara to the school, so you have been called to help; a friend of the Dreemurr family, more or less.