After months of insisting that they would definitely reach a nuclear agreement by November 24, and that there would be no extension of the deadline, the Iranian and world-power negotiators gathered in Vienna announced that there is no agreement and talks will be extended.

The new deadline is for a political framework agreement by March 1 and a final agreement by July 1. That's a real setback, and a bad sign for the feasibility of ever reaching a deal. So what happened? Here are three major reasons why the nuclear negotiations failed to succeed, and why reaching a deal has been, remains, and will continue to be so difficult.

1) Lack of trust makes some of the negotiation issues almost impossible

Everyone agrees, in very general terms, what a final deal should look like: Iran would limit its nuclear program such that it will not be able to build a bomb, and in return the US and world powers would greatly reduce international sanctions.

The big problem with this plan is it requires a lot of trust to make work. The US and world powers have to trust that, if Iran keeps some elements of a civilian nuclear program, it won't try to cheat on its promises and use those elements for a weapons program — something Iran has done in the past.

Iran, meanwhile, has to trust that the US and world powers aren't just scheming to weaken Iran's nuclear program and then keep the sanctions in place. That fear might seem strange to Americans, but Iran's leaders are positively convinced that the West is bent on Iran's destruction and will never tolerate a strong Iran, and thus that any promise of sanctions relief is a ruse meant to tempt Iran into surrendering its nuclear program. One reason Iranians believe this is the experience of Saddam Hussein's Iraq: Saddam abandoned his weapons of mass destruction and allowed humiliating international inspections, only to later be invaded over a WMD program that did not exist.

But this problem is much, much worse than a simple matter of "Iran and the US have to learn to trust each other." There are a few specific, mechanical negotiation issues that require one of the two sides to grant the other extraordinary trust: something neither of them is willing to do.

A good example of this is sanctions relief: does the UN lift its Iran sanctions outright or only temporarily? Iran wants the UN to pass a resolution lifting the sanctions outright; the US does not want to do this because, if Iran cheats on its commitments (which the US has a reasonable fear of Iran doing) then the US would have to pass a whole new set of sanctions from scratch, and there's a good chance China and Russia wouldn't support it. In other words, lifting the sanctions outright would give Iran an opening to grow its nuclear program and in the process be rewarded with reduced sanctions.

That's why the United States wants the UN to just pass a resolution every six months or so temporarily lifting sanctions. That way, if Iran cheats, the sanctions will eventually fall back into place on their own once the resolution expires. Iran hates this idea because it would leave the country totally exposed to the US; the Americans could increase their demands every six months and Iran would have little choice but to comply, or a new president could come in and simply refuse to continue passing the temporary resolution. At that point, Iran would have surrendered its nuclear program — its best defense, it believes, against American aggression — and opened itself to more bullying than ever.

And that's just one issue where the long history of mutual distrust makes actual agreements very, very difficult. There are more, and none of them is much easier.

2) Both sides are weakened by hard-liners at home

US Secretary of State John Kerry and his Iranian counterpart, Foreign Minister Javad Zarif, can't just lock themselves in a negotiating room and hammer out a deal. Both sides have to think about what sort of deal they can sell back home. And that, in many ways, is the hardest part.

In the US and Iran, political hard-liners oppose negotiations, want to set much higher demands for the other side, and have the power to blow up any agreement they don't like. In Iran, that means the hardliners in the country's parliament as well as the powerful Revolutionary Guard Corps, who can pressure the real national authority — Supreme Leader Ali Khamenei — to reject an agreement. In the US, that means members of Congress, predominantly but not exclusively Republicans, who see themselves as Iran hawks.

So any nuclear deal has to be able to win approval in three different places: in the negotiating room in Vienna, among the anti-American hard-liners in Tehran, and among the Iran-hawk hard-liners in Washington. Finding a deal that satisfies all three sets of requirements is really hard, and maybe not possible.

Just as bad, the mere fact that these hard-liners exist weakens Kerry and Zarif; both sides in the negotiations know that their counterpart is not the real, final authority.

That, as much as anything, seems to have done in the negotiations. The New York Times reports, as an example, that, "Zarif, while friendly, outgoing and Westernized, had pushed to the very limits of his brief; he often warned that the final decision would be in the hands of the supreme leader, Ayatollah Ali Khamenei." Meanwhile, "Kerry's position was complicated by the Republican midterm election victory and the fear of feeding the narrative that Mr. Obama was a weakened president."

Worse, both sets of hard-liners seem to be actively working at times to sabotage negotiations, and they're very good at it. The Republican-controlled House of Representatives is itching to pass a new sanctions bill, which would badly undercut President Obama's ability to negotiate and make him look powerless. When the Republicans take control of the Senate in 2015, they might just get it passed. In Iran, the supreme leader has at time announced increases to the nuclear program that have apparently taken Zarif by surprise.

3) They just don't agree

The trouble is not all just about domestic politics and difficulties in technical implementation. At some very basic levels, the two sides do not agree over the most fundamental issue of all: how much of a nuclear program Iran would be allowed to retain.

"Underneath all of this are differences in perceptions of what a reasonable, exclusively peaceful nuclear program in Iran should entail," George Perkovich at the Carnegie Endowment for International Peace wrote recently. "Iran insists on retaining certain capacities that others judge to be beyond what is necessary, and therefore suggestive of a desire to perhaps produce nuclear weapons in the future."

The US and Iran broadly agree that Iran should be allowed to have some sort of basic, peaceful, civilian program, but not a weapons program. But they can't, or simply don't, agree on what sets the line between a basic, peaceful, civilian program versus a weapons program. So there are lots of big and small technical disagreements, such as a new type of centrifuge Iran wants to start using, that express this disagreement.

This problem feeds the other two. Because neither side really trusts the other, each sees every technical dispute as not an honest disagreement but as a cynical, clandestine effort to cheat. And, meanwhile, the hard-liners in Washington and Tehran believe these disagreements make the deal vulnerable, and draw attention to those issues to try to push a deal to collapse. All of which does, indeed, make the deal more fragile and less likely to succeed.

It's possible that Iran and world powers will manage to resolve these issues in the months before their next deadlines, for a political framework deal in March and a complete deal in July. But it will be really, really difficult.