S.A. node is pacemaker to hea because -

SA node is the pacemaker of the hea because its resting excitability is the highest of all the nodes In healthy heas, these cells are found in the right atrium and are called the sinoatrial node (SAN; see below for more details). This action potential passes along the cell membrane causing the cell to contract, therefore the activity of the SAN results in a resting hea rate of roughly 60-100 beats per minute Ref Harrison 20edition pg 1456