King of the Romans, King of the Germans, etc... these were all titles used by the Imperial family, as I understand it. While under them were things like Dukes. Except for Bohemia, which was not a Duchy like, say, Saxony, but rather a Kingdom. Why?
This has been answered several times before.
This one is good.
And this answer by /u/dsmid from this thread, really summarizes it nicely.
In essence they agreed to pay tribute early. Those that refused eventually just became German states (Sorbs and Wends across the Oder, all the way to the Baltic states with the Teutonic States). That is a HUGE oversimplification.. just a way I made sense of a huge Slavic 'bite' taken out of the German and Austrian Empires.