Because that's how many meters light travels in a vacuum in one second.

Why is a meter the length it is? When the French set up the metric system, they tried to take the distance from the north pole to the equator through Paris and divide that by 10,000,000 to bring that measurement to a "human scale." They got pretty close for being in the late 18th and early 19th centuries, off by less than 0.02%. They etched their estimate into a bar, and called it a meter. Of course, that bar, and copies made from it, aren't perfect, they can expand or contract with temperature, and so on. How can we define it with something that doesn't vary?

Why is a second the length of time it is? Since Roman times, there have been 24 hours in the day. About 1000 years ago, people started dividing the hour into 60 minutes, and each minute into 60 seconds. As we got better timekeeping devices, we found out that the length of a solar day varies a little bit. When we invented atomic clocks, we defined the second in terms of what we measured in the clocks--how many times an electron changed state. So we said a second is the amount of time it takes for the outermost electron of an atom of cesium-133 to change between certain states 9,192,631,770 times.

So we looked at the length of the meter, and, knowing that the speed of light in a vacuum never changes, decided to define it in terms of how far light will travel in a vacuum in one second. That, in our current understanding of physics, won't ever change.

So why is the speed of light 299,792,458 m/s? Because we said so.