The reason for the timing of the strike, which has begun while negotiations were in progress is this: regime decapitation is more achievable  before the leadership has transferred to the most hardened facilities.

There is also a more general advantage in the reduced readiness posture of the adversary, and the disabling of launch sites before they can fire.

See Iran Protests; Is Revolution Nigh?

and What Are the Mullahs of Iran Thinking?