Why did Wilhelm I (or Bismarck) decide to perform official Germany unification in France?
In my opinion, if the ceremonial was to honour the Prussian king and be a tribute or allegiance to him, it should be held in Berlin/Potsdam (like cardinals honour new pope in Rome).
Of course, capturing Paris in 1870 war was a spectacular event showing the Prussian dominance, however (as it seems) the French capital was not considered to be kept by the Prussians (or Germans) forever.
I performed some research, however I couldn't find any detailed information, nor any trustworthy sources.
Bismarck's final step to complete unification was to challenge the power of France on the southern border. Since Richilieu and Louis XIV, France had made a divided Germany a prime component in French foreign policy. Bimarck would have to tread carefully if he were to unify the scattered Germans. [source]
This above suggest it was to show France that "what Richelieu and Louis XIV did" was now aborted. The best place to show something to France would be Paris.
This paper in some way suggest the creation of the German Empire is related to Charlemagne:
The Franco-Prussian War of 1870-1871 was reflective of the conflict between the western part of the Charlemagne Split (France) and the eastern part of the Charlemagne Split (Germany). The powerful German state of Prussia unified all of the German states and built the Second Reich (...)
Maybe it was because everyone was in hurry, and Wilhelm was visiting the General Staff. The idea might be backed by the webpage of the Versailles palace:
On 16 December 1870, a delegation from the Parliament of Northern Germany arrived in Versailles. It came to beseech the king of Prussia to accept the title of Emperor of Germany. The Confederation was dissolved on the 20th. The proclamation of the Empire was fixed for 18 January 1871 in the Hall of Mirrors. An altar was set up here for the religious ceremony. A stage was installed along the side next to the Salon of War, facing the spot where the throne of Louis XIV stood. 600 officers and all the German princes were present except Louis II. After the Te Deum, Bismarck, in his cuirassier’s uniform, read out the proclamation. When he had finished, the Grand-Duke of Baden shouted “Long live his Majesty the Emperor William!” The room rocked with the assembly’s “hurrahs!”. The Chancellor had finally made his dream come true under the paintings of Le Brun glorifying the victories of Louis XIV on the Rhine. He had also achieved his revenge for the defeat of Iena in 1806. The Germans soon left Versailles to the elected representatives of defeated France.