Mostly because one functional atomic weapon (I.E. not fission, fusion) could wipe Israel of the map.
AIUI (and I may not) it's not going to be a fusion weapon. Uranium enrichment is the name of Iran's game, not a Telemark-style heavy water production facility.
Still bad enough, nuking
anybody with a fission weapon. (Sum Of All Fears, anyone? Book version, please, the filmic 'reboot' of the Jack Ryan saga lacks something...)
I think the main thing is that Iran is doing Iraq-style brinkmanship... but are better at keeping it on the brink rather than forcing (or giving a good excuse for) direct action against itself. Quite a bit helped that there's now a phobia against such action, thanks to that little dalliance in their neighbour, but perhaps also by the fact that it actually appears a more certain threat. NK are also good at that. And for much the same reason insofar as they can at least keep up the appearance of (if not be
actually) being crazy enough to tip things over if their bluff is called... Not that they could hurt (much of) the US, with what they've got, but it effectively took just one pistol shot to start WW1, and it's a good bet that while it
could just be one, small shot, it'll be a helluva lot more power in the opening event.
Essentially, I think the public in the US (and allied countries) fear Iran because they know that Iran dislikes the US/UK/allies-thereof and they
might just do something about it. Gone are the days where it's effectively the national policy of the Soviet Union to hate America so much (indeed, in most of what supersedes the USSR, the US is a better friend than a number of nearer countries, some of whom
were Soviet, at one point). China doesn't have too many reasons to attack several of its best customers, even with its ability to do so. All in all the 'old foes' are only slightly more likely to attack as Canada is to march across the 45th parallel and have another go at burning Washington DC.
'Rogue states', though, are always a concern. In the days of asymmetric warfare, who knows who is nibbling away at who-else's infrastructure? From cyber attacks to seemingly independent explosive 'outrages' within a country. And not just covert operations, but outright False-Flag ones, and possibly even
False-False-Flag operations... But these are issues that the higher-ups in Langley and Thames House and whereavyer have to deal with. The information as distilled to the public is that Iran Is A Threat
TM. And there you're subject to various smudging and blurring of concepts that made it Popular Opinion that Saddam==Osama. A clearly incorrect viewpoint that was probably encouraged when it helped grease the wheels, by various people in charge of making those wheels of public opinion turn in the right direction.
I have no doubt that Iran
could be a threat. I wouldn't label them (or, more accurately, the controlling hierarchy) as Kill On Sight because of this, but at the same time it'd be a brave President/CIA director/whoever to
not consider all the options needed to "get the retaliation in first". Put them on paper, at least. There's far less likely things been considered (historically, consider
War Plan Red, or something else from the
rainbow, and I bet there's something more contemporaneous sitting in a federal vault, somewhere).
In the end, if it comes to war, it will be because (to paraphrase Captain E. Blackadder, HoUG) it would have been too much trouble
not to go to war. It's not inevitable, of course, but Interesting Times are certainly a risk given how the main parties are dealing with each other, and my probably-biased western POV tends to want to put the blame upon the Iranian high command (or perhaps the power behind their collective thrones)...
Anyway, that's my take, and doubtless wrong in key respects.