Because FM is a newer and better technology:
Imagine the radio waves are light. AM transmits by changing the brightness of a light (Amplitude Modulation), FM transmits by changing the colour of the light (Frequency Modulation).
If you have a clear view of both lights then they both sound good, but imagine if you’re driving and trees get in the way of the light beam. You can still see the light through the leaves, but not as well.
With AM the brightness of the light keeps changing as leaves block it so the volume keeps going up and down. With FM it doesn’t matter how bright the light is because you only care about the colour, which doesn’t change.
The reason you still have AM as well as FM because AM is much cheaper to broadcast and the signals travel further.
This is the best explanation I’ve ever read about the difference between AM and FM.