Because it's a fantastic starter VN. It doesn't rely on the typical Japanese bullshit tropes, sure, there are plenty of elements there, but it almost never shoves it into your face like so many Japanese visual novels do. It manages to remain believable throughout the story, which is something few VNs manage to do. In this regard it does benefit form the fact that it remains pretty realistic, but that not necessary for something to be believable. Majikoi is a good example of a unrealistic visual novel that remains believable throughout the story, right from the start it sets you up with ridiculous scenarios and remains true to its universe. Katawa Shoujo also has an underlying theme that is very easy to relate to and make you feel for the characters, a very fitting soundtrack, decent art and some of the most human characters I have come across in a visual novel. And it's truly free.