Azadeh Moaveni brings up a little-discussed reason:
The hard-line political forces in Tehran most opposed to a nuclear compromise with the West also dominate the institutions—the Revolutionary Guards, the judiciary, and various security bodies—that perpetrate the most serious rights abuses, ranging from summary executions to the detention of journalists, religious and ethnic minority activists, and Iranians with connections to the West. For most of the past decade, these hard-liners exploited times of tension with the West, such as periods when the threat of a U.S. military strike was amplified, or when Iranian nuclear scientists were being assassinated. For the hard-liners these were opportunities to crack down on regime critics, and expel them from universities, newspapers, government ministries, and city councils.
The fear among Iranian dissidents is that a breakdown in nuclear talks would prompt another wave of repression. Inevitably, a breakdown would be seen in Iran as the West having rejected reasonable Iranian overtures (just as the West would see it as Iranian rejection of reasonable Western overtures). Hard-liners would depict this rejection as more evidence of Western disrespect, even contempt, for Iran, and would try to exploit any sense of renewed tension to push their oppressive agenda. That would be especially easy if threats of a military strike by the United States or Israel were revived.