Seems that the rate of PSAT taking is regional. Based on numbers of PSAT takers and numbers of students in each high school class level, some states may have over two thirds of high school students taking the PSAT (e.g. MA), while others may have only about a fifth (e.g. MS).
It would not be surprising if the rate of PSAT taking correlates to SAT versus ACT use, since people may perceive the PSAT as a “practice SAT”, so those focusing on the ACT may be less aware of it, or its National Merit implications.