I think you will find that the custom of granting the Bailiffship of Honour and Devotion was more widespread than you seem to think. In the period from 1870-1914, not only every Catholic sovereign except the king of Italy was given the Order, but also Edward VII and Emperor Nicholas II of Russia. They did not "join", but were honoured with the cross. More recently both Queens Juliana and Beatrix, despite being Protestant, were given the Grand Cross, as has Grand Duchess Maria Wladmirovna, her son Grand Duke Georg, and the Crown Prince of Serbia. In the House of Austria, from the late 18th century there was always an Archduke who was Hoch und Deutschmeister (GM of the Teutonic Order) and others who were Bailiffs of Malta - the Grand Priories of Austria and Bohemia were important institutions of the Order.

--Previous Message--
: In the early twentieth century the membership
: of the Sovereign Military Order of Malta was
: largely from noble families - but there were
: relatively few members from royal families
: (whether reigning or formerly reigning).
: Today things are somewhat different. Among
: the Bailiffs Grand Cross of Honour and
: Devotion are King Albert II of Belgium;
: Franz, Duke of Bavaria; Louis Alphonse, Duke
: of Anjou; Hans-Adam II, Prince of
: Liechtenstein; and Carl, Duke of
: Württemberg.
:
: Does anybody know why it is that the
: Archduke Carl Franz Joseph (later Emperor
: Karl) joined the Sovereign Military Order of
: Malta?
: