In many countries, the parties select candidates internally and then present them to the public. Letting non-party members vote or register just before voting would be unthinkable there.
Then again, this is probably a necessary result of the US political system: majority voting rules, therefore (basically) only two parties, and of course, the US is huge and diverse.
The whole process is ridiculously long and expensive. But it's also a way to get people familiar with the candidates early on.