According to Gregory of Tours, Clovis (the king of the Franks) converted at the request and insistence of his wife Clotilde. During a difficult battle at Tolbiac (in 496), the Franks fought off an Alamanni attack with difficulty. During the battle, Clovis is said to have implored God's help in exchange for his conversion.
At the same time, the Franks were attracted to the very rich Aquitaine, which was in the hands of the Visigoths (who were not Christians). This would give them the support of the Christian population of the region and would probably make them more acceptable as new rulers. (Source: M. Univers social