Answer
This is because of the Heisenberg Uncertainty principle for matter.
Work Step by Step
The Heisenberg Uncertainty principle for matter states that $\Delta t \Delta E \geq \frac{\hbar}{2}$.
Laser light comes from a system that remains in a long-lived metastable state (larger $\Delta t$), so the uncertainty in its energy is very small (smaller $\Delta E$), i.e., it is more monochromatic than ordinary light.
