Because the French fleet didn't want to join the UK. The French State (more commonly known as Vichy France) wanted to try and walk a fine line between collaborating with Germany while still remaining independent. If the Vichy government had let their flees defect to the British the Germans would have been very angry, and Vichy France's survival depended on keeping Germany happy. For their part, the French promised the British that there fleet wouldn't fall into German hands, but that wasn't a good enough guarantee for the British.