Because it's better than not having a deal in place, which has been the case up until now. It reels in the Iranians, places firm oversight on their programs. Simple as that. It's better than nothing, and nothing is a free pass to develop nukes. This makes it very difficult for them moving forward, and places stringent controls on the process. I really don't understand why anyone would be opposed to this.
All one needs to ask to see this agreement for the sham that it is....... is who will inspect them and hold them to this "deal"?