The speed of the light travelling is faster and that's why the flash is seen first and after a while the sound is heard. The same is with aeroplanes. Where the sound of the plane is coming from, the plane is always way ahead of it.
This is one simple reason behind this, the speed of light is far too fast than speed of sound which is limited to almost 343 meter apx per second compared to 3,00000 km of light's speed.
