Queen Elisabeth of Belgium was previously Duchess Elisabeth in Bavaria. I do not understand why Duchess is used. Was she not a Princess? Or is Duchess a Bavarian equivalent of Princess?
The title Duke in Bavaria (not of Bavaria) was created for Wilhelm of Wittelsbach, Pfalzgraf of Pfalz-Gelnhausen (1752-1837). He married Maria Anna Pfalzgräfin of Pfalz-Zweibrücken (1753-1824).
Maria Anna was the sister of Karl II. August Duke of Pfalz-Zweibrücken (who was in love and vice versa with Archduchess Maria Amalia of Austria, but she wasn't allowed to marry him and instead was betrothed to Ferdinand of Parma. He became then very anti Habsburg and married Maria Amalia of Saxony) and the late King Maximilian I. of Bavaria.
When Maximilian became Elector (Kurfürst) of Bavaria, Wilhelm was made Duke in Bavaria (but it was just a honorary title) and received the style Royal Highness as he was just from a side line of the Wittelsbachs. This was in 1799 and was one done to unifie Bavaria (so Wilhelm wouldn't one day come up with the idea that he could rule Bavaria instead).
Pius, his only son and successor (1786-1837) would marry Amalia of Arenberg and they became the parents of Duke Maximilian in Bavaria (and he the father of Empress Elisabeth). When Princess Ludovika was forced to marry Max she felt betrayed and like she had to marry beneath (which was true).
The title Duke/Duchess in Bavaria is simple a sideline (non reigning one) as opposed to the Kings (or Electors) of Bavaria. The children of the Kings of Bavaria were always Prince/Princess of Bavaria.