Because the French did not surrender totally to the Germans. A 2/3 slab of France was occupied directly by the Wehrmacht while a de jure (abit a puppet state) French government was allowed in the southeastern 1/3 of France. The new government was the Vichy Republic. Technically, all military forces formerly under the French government in Paris were transferred to the new Vichy France government in Lyons (I think it's Lyons). Because of that transfer, all French sailors were now commanded by what was then a legal state (technically). Because of that, any men or ships defecting to the UK would be traitors. This pressure forced most of them to stay. Also, the majority of the French fleet left was based in the Mediterranean. If a ships captain decided to flee, he would have to make it to a British base nearby. So, Malta or Gibraltar. Neither of which were close, and both routes to safety involved going by either Italy or Spain, fascist countries. In any case the choice was only there for a short time. Soon after the fall of France, the fleet was either scuttled, taken forcibly by the Germans to deny the Brits extra ships, or attacked the British to deny the Jerries extra ships.