Because of copyright law. Check out the Copyright Basics for Wikipedia Edit-a-thons page of this guide for succinct and important explanations of what copyright is and what it means for Wikipedia contributors.
You retain copyright to your original materials (both text and other media) that you create and contribute to Wikipedia. This means you can later republish and relicense the materials, but you can’t retract or alter the free license for the copies of the materials you put on Wikipedia. For more, see: Wikipedia: Copyrights.
If you are using someone else's text or image pursuant to a Creative Commons or similar license for reuse, be sure to comply with the terms of that license. This could include, for example, attributing the author, sharing it under the same terms as the original license, etc. Find out more about Creative Commons licenses.
There are legal issues to consider beyond copyright law that affect whether you can upload or repost an image to Wikipedia. For instance, if you took a photo of someone in a private setting, you may need the person's permission to post that photo since it could impinge upon the person's right to privacy. Wikipedia also precludes use of images that are defamatory, demeaning, or violate human rights.
For more on Wikipedia's privacy policies, see: Wikipedia: Image Use Policy - Privacy rights.
For more on privacy in scholarly communications generally, see: Non-Copyright Policy Concerns: Privacy.