Because of the troubles that occurred in Paris, namely the Réveillon riots, and the growing agitation of Versailles, Louis XVI decides to convene the Estates General in Soissons, near his newly renovated castle of Compiègne where he would stay during the deliberations.
(The castle of Compiègne is situated 142 km north of Paris)
The safety of the King and the security of Estates General has to be insured by units of la Maison militaire du roi de France. Unfortunately due to the situation in Paris, decision is taken to keep the turbulent troops of the Gardes Françaises in the capital to maintain order, they won’t be allowed to follow the king in Compiègne and Soissons. Les Gardes suisses and a few foreign regiments are selected for this difficult mission.
On 5 May 1789, the Estates-General convene in the town of Soissons...
Edit : Why Soissons?
According to the Journal des États Généraux (published minutes of Estates-General), in june 1789, the king suggested to move the Estates-General in Soissons or Noyon.