Etymology of pan /pani

that explains why the common Slavic form isn't used

that explains why the common Slavic form isn't used
could you explain?

that explains why the common Slavic form isn't used;

There is no such form as common Slavic .

There was a poor guy Pantofel.

Send a script to Hollywood and they call you

marqoz: According to Aleksander Brueckner, polish etymologist, pan has its origin in old word żupan meaning a tribute gathering officer or administrator of some territory called żupania. The word was known in Czech, Croat and Hungarian (ispan). Later the word was shortened and simplified to pan.

Translated:
Żupan, a very old word, had two separate meanings in Old Polish language. Naruszewicz says that Czechs, Poles and other Slavs called the country lords "żupans". A Czech chronicle of the 1109 attests that such name was given to officials in Czechia. Some say that in very ancient times the nobility in Poland was called żupans. (...). In a word - żupan meant a wealthy man, a dignitary, and today's "pan" is only a shortening of "żupan". But this shortening is not just a product of recent centuries, because in a document from 1257 we find: "Thomas qui dicitur Staripan." In other Slavic tribes żupan was used to describe: judges, mayors, governors, and the like superiors.
(...)
A dignitary name żupan was once widespread in Poland. Żupan's wife was called "żupani" and this word has been adopted from Poles, by Prussians, to whom it meant the lady of the house. The word "żupani" in Prussia and Lithuania - as says Brückner - was adopted from Polish when inhabitants of towns and castles still bowed down to żupanis.

The importance of a county as "ziemica" and żupan as its supervisor, a mayor, is changing in nature to a kind of tribute (tax) and an official collecting it. Besides the latin word "zupanus" there also appears "zuparius". When later żupy and żupniks (says Brückner) are becoming limited to benefices and salt mine offices, "zuparius" (żupca in Polish) becomes a low clerk of a court, as we can see in the Mazovian Act of 1406. Czechs stopped using the words żupan in the sense of high dignitary at the end of thirteenth century.