Because it just seem awfully convenient that Iran would allow foreign investment to build up the country and then just like that, they can end the deal and resume working on weapons but with a more modern country in the aftermath. Also I was wrong when I said 25 years. They can literally start nuclear associated activity again in 2025
http://www.bbc.com/news/world-middle-east-33521655 Here guys seriously, this isn't even NYT, read the deal.
There is a chance that the Iran regime would change and the people in the country would rise up.
But think about the risks as well described in these two scenarios:
1. Keep deal, modernize and invest in Iran. Iran ends deal and resume Nuclear Program and develop a weapon within 3 months, maybe sooner because I'm sure they'll develop better infrastructure in that time (people said they can make a weapon in 3 months in this thread?). They get their cake and eat it too.
2. End deal now, let Iran decide what it will do. If they decide to make the weapon it'll take 3 months (according to people in this thread), but at least they didn't get any foreign investment to prop up the country. Deal with the conflict 3 months from now, which Iran almost 100% wouldn't follow through with. OR they can come back to table and negotiate a deal that would not resume the programs/more limitations on development of missiles.
So really do you want to do deal with the inevitable conflict now or later? History says that leaders tend to pass off the problems to later generations