Basically because there was very little chance that the ships would fall into Axis hands in the near term. After the French surrender the fleet was one of the few chips the French still held and they were not about to simply hand it to the Germans, or indeed the British. In addition the French naval commanders had given personal assurances (to the British) that they would obstruct any German attempt to seize the fleet. Not unreasonably, they felt that their loyalty was to their government and that they had given their word to their former allies. Sadly the British government didn't feel the same way.

Furthermore, the ships at Mers-el-kebir and Oran were not really at risk anyway since the Germans would have had to mount a seaborne invasion of Vichy North Africa to get at them. Hardly an event that the Vichy government would have tolerated.

The Germans did attempt to seize the French fleet at Toulon on 27 November 1942 after the Torch landings. The French scuttled almost every ship of military value and several submarines were able to cross over to the allies.