Why were Germany and Switzerland the birthplaces of the Reformation-what made them vulnerable? I'm pretty sure it was because they didn't have an absolute ruler?
The ideas of the Reformation existed to some extent all over Europe. So why did the Reformation start where it did? I think that you are right and that the lack of strong central control made Germany and Switzerland fertile ground for these ideas to take root.
The Reformation began for the most part in free cities in Germany and Switzerland. These cities were officially ruled by the Holy Roman Emperor and not by any other lower prince like most places in Germany were. In practice, many of these cities were actually quite independent, largely because they were economically important.
Because these cities could not really be controlled by any outside political forces, it was more possible for the Reformation to take root there instead of being squashed by an absolute ruler.
Do note, however, that once the ideas of the Reformation took hold, they spread even to places that did have fairly strong rulers. Many of these rulers found it useful to embrace Protestant ideas.