Because the atmosphere in the home is quite chilly. John has committed adultery with Abigail, and though Elizabeth does not yet know it, she suspects something is wrong.
In John's mind, it is all over and he wants his family and his wife back. He foolishly hopes that "dressing up" the house, pretending everything is fine and can go back to normal. Bringing in the flowers, and spring, a time of new beginnings, is symbolic of his hopes.