Because of the potential global catastrophic dangers from nuclear weapons. Since WWII there have been a series of global non-proliferation and inspection treaties signed by most countries in the world that can & will be backed up by international sanctions like on DPRK and to a lessor extent Iran are under, with their flouted ban on ballistic missile development. Like it or not up to WWII the UK was the unliked global police force & since then it has been the unliked US. In both cases as it has reined in many nations against the wars & geopolitical gains they would like to achieve. The post-WWII series of treaties & our current global rules based system culminating in the Helsinki Final Act. To further reduct the danger of MAD global destruction there have been a series of START treaties between the two main nuclear powers, the US and Russia.
Eg. The sanction against Russia on the annexing of Crimea are an example of punishment given by the West for breaking mutually agreed international treaties and rules.
It is in none of the countries who are in the nuclear club to allow unstable regimes to acquire nuclear weapons and use them to blackmail their neighbours for geopolitical gain or to start launching them around the world if they think it will save their regime. Imagine if General Galtieri with his Falkland invasion if they backed it up with ICBM nukes launched at the UK to save the Falkland's defeat and the falling of his junta.
The fact that we could & probably would have relatiated, so you have two countries suffering horrific damage and casualties, nuclear fallout & maybe a short nuclear winter would also not be appreciated by us or them or the 200+ innocent bystander countries and this is before you start considering nuclear weapon security, terrorism or weapon safety, so they aren't accidentally or deliberately launched due to poor control procedures, design or maintenance.
We have just had the murder of 179 passengers as a result of poor training & control of the military base air-to-air defence systems, now imagine if those itchy fingers were on a nuclear ICBM button & central
London Barnsley were the landing coordinates entered into it.
Something doesn't add up on the shooting down of the UIA passenger jet & I'm hearing some ugly rumores at the moment on why it was targeted, but nothing that as yet has been substantiated or independently verified, so I'm not going to repeat them at this time.