Gotta go for today...
Other countries have had left and right governments and yet signed on.
I imagine that the answer as to why they haven’t signed on is contained in the final part of your post.
Israel would probably say they have no nukes, and Iran would probably say the same in reply. Around and around she goes, where she stops...
Then perhaps Israel would say she has no secrets - so why any inspections - and Iran would probably say the same in reply.
It is going to come down to the US getting a good deal with Iran, getting no deal and we do nothing after that, or we go to war.
If we go to war, you can bet it will take a half a million boots on the ground to secure the country and get to the underground areas scattered all over Iran. Who wants that?
One nuke (of any size) on the ‘Holy City’ of Qom, and one conventional cruise missile up the butt of Ayatollah Khamenei, the Saudis take note and radical Islam isn’t heard from again for a couple generations. Western governments don’t understand the power of symbolism in Islam.