I don't know officially, but it could be because blurred photos are of limited use. Plus, even though you've named it 'blurred', it would take a lot of time and deliberation for every photo to decide if a photo is meant to be blurry, or just has poor focus. Also, it is very easy to blur a photo in any photo editing software, so if people want a completely blurred photo they can take any in focus photo and do it themselves. They seem to like minimal post-processing or effects on photos you upload, preferring them to be something with broad appeal that people can do their own effects to later.
... View more