Iran voluntarily signed up to the treaty and received various benefits for doing so in return for agreeing not to pursuing nuclear weapons capability (same as every other non-nuclear weapons state signatory).
That is of course true, but also a legalistic take, as Iran not signing on to the NPT, or indeed leaving it, would not make Western powers go "oh ok, so it's fine now".
The reality is that most of the non-nuclear states are largely happy with the arrangement of living within the US nuclear umbrella and acknowledge that more nuclear weapon states do not benefit the common good, the nuclear weapon states have no interest in losing their monopoly (and are arguably breaking the treaty by failing to pursue disarmament).
The difference between Iran and North Korea is that the political system in Iran is much more nuanced, and makes it much harder for them to pursue a "lets acquire a nuke at all costs" policy, so they have to play it smart, and acquire the ability to build a weapon without actually doing it.
The difference between Iran and say India is that both the US and Israel justifiably see Iran as an antagonist, and in addition Israel is able to command significant political capital in Washington.
- Israel justifiably sees Iran as an antagonist, and they have a significant influen
As far as I know, India recognizes Pakistan's right to exist, and doesn't regularly threaten to wipe Pakistan off the map / drive all of its citizens into the sea.
