Eli5: Why is it so hard for a country to make a nuclear bomb?

Question

I’m assuming the science of making one is out there. Why then countries like Iran who so want to develop atomic weapons haven’t been able to do so?

In: Physics

Answer 1

Science isn’t hard, getting the fissile material is the most challenging part from actual execution point of view.  Many countries could develop a nuke almost overnight due to having all the infrastructure in place – see concept of nuclear latency: https://en.wikipedia.org/wiki/Nuclear_latency 

Political consequences and harassment from other states is main reason that we don’t have more nuclear states.