is because the French controlled the country for nearly 60 years.
From wiki:
In the 19th century, contact with Europe began with the first European explorers—notably Monteil (French) and Barth (German)—to travel to Niger.
Following the 1885 Berlin conference during which colonial powers outlined the division of Africa into colonial spheres, French military efforts to conquer existing African states were intensified in all French colonies including Niger. This included several military expeditions including the Voulet Chanoine Mission, which became notorious for pillaging, looting, raping and killing many local civilians on its passage. On 8 May 1899, in retaliation for the resistance of queen Sarraounia, captain Voulet and his men murdered all the inhabitants of the village of Birni-N'Konni in what is regarded as one of the worst massacres in French colonial history. French military expeditions met great resistance from several ethnic groups, especially Hausa and Tuareg groups. The most notable Tuareg revolt was the Kaocen Revolt. The French authorities also abolished the widespread slavery among Tuareg communities.
By 1922, all resistance to colonial rule was eliminated and Niger became a French colony. Niger's colonial history and development parallel that of other French West African territories. France administered her West African colonies through a governor general at Dakar, Senegal, and governors in the individual territories, including Niger. In addition to conferring a limited form of French citizenship on the inhabitants of the territories, the 1946 French constitution provided for decentralization of power and limited participation in political life for local advisory assemblies.
The end of the colonial era was characterized by a transformation of the political environment in French West Africa and Niger. The Nigerien Progressive Party, the Nigerien section of the African Democratic Rally Party, founded in May 1946, united various tendencies of Nigerien people in the movement for national independence. In alliance with progressive French elements and other independence African movements, the movements acquired the suppression of forced labor and arbitrary requisitions as well as legal equality between the African and the French citizens.
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