Iran apparently gave up very little to get President Obama to transfer the cash, unfreeze the assets and lift the sanctions. So far Iran has said it would honor the agreement, but has hinted that it could restart it. If Iran really had peaceful intentions, would it threaten to resume its program? There is a long history of dictatorships violating arms agreements, and no one should be surprised that Iran continues that tradition. Dictatorships usually will not voluntarily restrain themselves.
Several other countries were parties to the deal, and they want to keep it - Russia and China, who seldom operate with the free world's interest at heart; and France, the UK and Germany, who are staunch allies but who also are home to many companies that have lucrative financial deals with Iran. But the countries Iran has threatened with destabilzation or terrorism are pleased the deal has been revoked.
The Obama Administration valued the agreement more than it valued the goal of a nuclear-free Iran. The Trump administration places greater importance on a nuclear-free Iran than it does on the existence of an unenforceable agreement that probably did not curb Iran's aggression or limit its weaponry. Let's hope that President Trump is able to arrive at a deal - better yet, a treaty - that does both.