Why?, This is because it's not gonna be easy to have a global ban on Bitcoin mining happen at same time. reason, this country may find reason to ban Bitcoin mining while the other country may see reason to adopt or see an opportunity to continue mining. Taking for instance, China banned mining while US took the opportunity and their mining firms.
Such measures don't happen over night, you are right. They are spread out over time and take years if done systematically. Let's say that's the direction the US takes, who could follow them? The UK is the obvious culprit. Maybe they get France on board and Germany. The last three are not that important in mining circles, but they are in global politics. Once you do that, politicians do what they do best. Intimidation, demands, blackmail...
Oh, you want a loan to fix your economy? Well that is going to be difficult when bitcoin is being mined in your country. Maybe there is something you can do for us first...