Answer:
Because depicts the Annunciation in a indoor ambience, instead of be portrayed in a landscape as it was usual in the 14th and 15th centuries.
Step-by-step explanation:
It's a masterpiece with several symbolic elements. For the first time in art history, the Virgin Mary is portrayed indoor, receiving the Angel. This piece of art is divided into three parts, each one depicting the event from different points of view. At the center is Mary and the Angel. For the first time in an artistic depiction, she is not praying when the Angel shows himself, instead, she is reading. This was the way found by Robert Campin to say that she was on her day by day deals when the Angel Gabriel appears. Actually, her expression did not show any surprise. Maybe the Angel didn't say to her, yet. On the right, there is St. Joseph, working. And on the left, a group of people visiting her. It was the first time that a casual and ordinary behavior was portrayed in a canvas such as this, with this theme.