SA node acts as a pacemaker of the hea because of the fact that it:

The sinoatrial (SA) node normally displays the highest intrinsic rate. All other pacemakers are referred to as subsidiary or latent pacemakers because they take over the function of initiating excitation of the hea only when the SA node is unable to generate impulses or when these impulses fail to propagate. There is a hierarchy of intrinsic rates of subsidiary pacemakers that have normal automaticity: atrial pacemakers have faster intrinsic rates than AV junctional pacemakers, and AV junctional pacemakers have faster rates than ventricular pacemakers. Ref: Chen P., Antzelevitch C. (2011). Chapter 38. Mechanisms of Cardiac Arrhythmias and Conduction Disturbances. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst's The Hea, 13e.