Because it isn't a French word. The car was named after André-Gustave Citroën who was a Jew who's heritage was Dutch and Polish.

It is an imported name, coming from "Citroen" ("lemon" in Dutch): "André-Gustave Citroën (5 February 1878 – 3 July 1935) was a French-Jewish entrepreneur. He is remembered chiefly for the make of car named after him, but also for his application of double helical gears. - Life: Born in Paris in 1878, André-Gustave was the 5th and last child of a Jewish diamond merchant Levie Citroen from the Netherlands and Mazra Kleinmann (of Warsaw, Poland). He was related to the famous British philosopher A. J. Ayer. The Citroen family moved to Paris from Amsterdam in 1873. Upon arrival, the diaeresis was added to the name, changing Citroen to Citroën (a grandfather had sold lemons, and had changed the consequent name Limoenman "lime man" to Citroen ("lemon" in Dutch). His father committed suicide when André was only six years old. André was a graduate of the École Polytechnique in 1900. During World War I, he was responsible for mass production of armaments. André founded the Citroën automobile company in 1919, leading it to become the fourth-largest automobile manufacturer in the world by the early 1930s. He died in Paris, France, of stomach cancer in 1935 and was interred in the Cimetière du Montparnasse, the funeral being led by the Chief Rabbi of Paris. In 1992, the Parc André Citroën public garden in Paris was named after him. In 1998 he was inducted into the Automotive Hall of Fame in Dearborn, Michigan." Source and further information: http://en.wikipedia.org/wiki/Andr%C3%A9_Citro%C3%ABn Many famous French people were originally foreigners.