Short answer: It had been decided that light will always move at a specific speed no matter who or what observes light and even time itself would bend to make that be always true.

Long answer: Let's start with understand a bit about relativity. Let's say you're on a train with a friend and your friend gets up and walks towards the bathroom. From your perspective you see your friend moving at about walking speed towards the bathroom. It's not particularly fast. But what about someone outside the train? Let's say a man is standing a safe distance away from the train tracks and your train zooms past this man. This man sees your friend who was walking to the bathroom. From his perspective, she is going really fast and is zooming past the man. Now your friend has 2 speeds. One really fast speed from the perspective of the man outside the train and a slow speed from your perspective. Both speeds are correct and it all depends on what the speed of the observer is. Everyone will have many different speeds just like your friend and that is what relativity is.

But here's the thing. Light is special because it will always have 1 speed: c! No matter who the observer is and how fast the observer moves, light will always move at the same speed(in vacuum) which is a specific number that is roughly about 300 million meters a second. Now obviously this would seem to contradict the earlier point about relativity but here's the magical thing that lets light move at c. The observers looking at light will either have their time sped up or slowed down to make it such that light is moving at c. So if you try to run really really fast to try and catch up to light, light won't appear to have a lower speed, it would still be moving at c by magic because time slows down for you and light will appear to go faster to make it so it moves at c.