Why is drag-and-drop image upload not implemented more often on websites?

I am not a developer, but I know that it is possible to implement drag and drop for image uploads using a dropzone on a webpage. There even seems to be a choice of code available to implement.

It has been around for a while, but still I have not seen it implemented very often. I am considering it for our own website for improved ease of use, but am a little cautious. Are there good reasons not to implement this? Perhaps it will not work on common browsers/OS/Java combinations? IE?? If that is the case, I would perhaps still want to implement it for our in-house custom CMS system. Any experiences?