Why is National Merit Scholar award based on PSAT instead of SAT?

My D’s school did not promote much the PSAT although they did offer it to 10th grader upon request in the past. Many students did not even know this offer. Now they are switching to the full gear PSAT for all grades due to the state policy. We will see if that change anything. It has around 20-30 NMSF/NMF (~5-8% of the class) every year before the change.