A charged particle moving at constant velocity will not create electromagnetic waves, but that same particle moving with a changing velocity (an acceleration) will create electromagnetic waves. Why? A.
Because electric forces and magnetic forces cancel out as a particle reaches a specific acceleration threshold.
B.
Because accelerating charges create changing electric fields, and the changing electric fields induce changing magnetic fields.
C.
Because only accelerating charges can repel their own electric fields, thus creating ongoing forward motion.
D.
Charged particles can accelerate only if they are in a vacuum, and since electromagnetic waves do not need a medium, they are created under vacuum conditions.