Because it was not originally defined as this. We needed a standard to measure distance, we chose a specific length and named it as metre. As time passed for sake of accuracy and convenience probably we changed the definition of metre quite a bit. Although they are different, they express the same amount of length anyway.
Speed of light is in vacuum 299,792,458 m/s.
Light travels a distance of 299,792,458 m in 1 sec.
Therfore for light to travel 1 m, the time required is 1/299,792,458 sec.
Since the speed of light in vaccum is a universal constant, it is opted as a standard for defining 1 metre in a more specific and accurate way.
When the SI system was established, In 1889, it was redefined in terms of a prototype bar composed of an alloy of 90% platinum and 10% iridium. The metre was defined as a distance between two scratches on the bar at 0 degree Celsius. Temperature was mentioned because length increases with temperature.
The newest defenition is more accurate than the previous one.
Maybe in future, if a more accurate defenition is possible, the current definition would be discarded.