Very shortly summarized, only if the particle is faster than the local speed of light, you get constructive interference between many atoms that were polarized by the moving particle.

For a slower than light particle, you also get emission, but it is completely random and thus does not give the well defined emission in a cone of Cherenkov radiation.

For a faster than light particle, the spherical suddenly line up to form a cone:

https://en.wikipedia.org/wiki/Cherenkov_radiation#/media/Fil...