The FM trick is that the available parameters that can be modulated through velocity result in a wider range of tonal variation, where traditional subtractive synthesis is usually simpler, and velocity changes affect parameters that don't result in wide changes to the tone, such as volume or filter envelopes.

With FM, modifying an operator envelope can result in a completely different sound. A well-crafted patch that takes advantage of this, played by a good keyboardist, will feel alive, in a way traditional synths of that time couldn't achieve.