
Are you sure?
AFAIK, in most countries ( Belgium being the only exception I know ), a Prince ( even if not of the Blood ) is considered above a Duke, in order of nobility. So I supposed a Principality was considered as more honorable than a Duchy, for the same land. Yes, there were some rather insignificant principalities ( Monaco and Andora come to mind ), but that was also the case for some Duchies.
OTOH, I admit I linked principality to independence, which may be a bend because of french history ( The title of Prince was reserved, in french nobility for the Royal familly, with a few exception, except for independent principality integrated inside France ).
Oh my

Firstly, the the assumption that the title of "prince" outranks the title of "duke" is completely unfounded. It is a case by case basis, depending on the local system of nobility. As a rule, one generally finds the title of "prince" at the top when there is no native title of "duke" present (such as in the Russian Empire).
Generally speaking, however, the title of "duke" outranks the
title of prince; notice I say "title" because the idea of a "rank" of prince (i.e. a prince of royal blood) is not included in this assumption. However, even the title of "prince" for the son of a king is far from consistent, especially in pre-revolutionary Europe (it didn't develop in England--consistantly at least--until George I brought it over from Germany, and it never caught on in France, Portugal or Spain at all).
Secondly, the assertion that in France, the title of "prince" was reserved for the royal family is completely inaccurate. The
title of prince never existed in France before the Revolution. While the
rank of "prince of the blood" (prince du sang) did exist for members of the extended royal family, this was
not a title. Period. Grandsons of a king of France and their issue were referred to as "N. de [fathers primary appanage]" followed by whatever other titles they possessed (both by law and by courtesy).
There
were some exceptions to this rule, however. Certain foreign titles of "prince" were recognized by the king of France (especially after Artois was annexed), but their ranking in the order of precedence was disputed until well after 1800 (when they were placed after dukes and before marquis in the official order of things). They certainly were
never believed to outrank dukes, however.
There were also some small feudal principalities existing in France, all of which (to my knowlege) had their origins in the Holy Roman Empire, and had simply been incorporated into French territory and recognized the French king as their liege (such as Monaco and Orange).
The
rank of "prince étranger" (foreign prince) also existed in France, and could be bestowed by the French king on subjects whose titles of prince were recognized in France (as stated above) or who were of royal descent (such as the Rohan and Guise families). This was, however,
not a title, but a rank, which carried with it many special privileges, though in practice, princes étranger ranked after peers of the realm and princes of the blood (many argued that they outranked non-peer dukes, thought that is another discussion entirely).
Finally, many nobles did
assume the title of prince, especially in the eighteenth century, as it was fashionable to do so, but there are many examples of unauthorized assumption of title in France, such as that of "marquis". For example, while the eldest son of the duc d'Elbeuf might call himself "prince de Lillebonne" the title remained legally that of "comte de Lillebonne" and all addresses to him as "prince" were essentially by courtesy, even those from the king. And while there were some instances of peculiar seigneuries in which the holder was titled as "prince" this was mostly in the (very rare) feudal instance, implying that he was directly subject to the king alone as vassal.
More information can be found
here. But essentially what I'm saying is that we shouldn't jump to conclusions about noble titles or make broad sweeping statements. Sorry to go off on a rant
