Why is Vaduz the capital of Liechtenstein?

In the 17th century the Liechtenstein family was seeking a seat in the Imperial diet, the Reichstag. However, since they did not hold any territory that was directly under the Imperial throne, they were unable to meet the primary requirement to qualify.

The family yearned for the added power a seat in the Imperial government would bring, and therefore sought to acquire lands that would be Reichsunmittelbar, or held without any feudal personage other than the Holy Roman Emperor himself having rights on the land. After some time, the family was able to arrange the purchase of the minuscule Herrschaft ("Lordship") of Schellenberg and countship of Vaduz (in 1699 and 1712 respectively) from the Hohenems. Tiny Schellenberg and Vaduz possessed exactly the political status required: no feudal lord other than the Emperor.

Thereby, on January 23, 1719, after purchase had been duly made, Charles VI, Holy Roman Emperor, decreed Vaduz and Schellenberg were united, and raised to the dignity of Fürstentum (principality) with the name "Liechtenstein" in honour of "[his] true servant, Anton Florian of Liechtenstein". It is on this date that Liechtenstein became a sovereign member state of the Holy Roman Empire.