It's related to age, as you would expect from any contagious virus that's ubiquitous in the environment and doesn't go away. 0.01% of newborns have HSV-1 [1]; 90% of 50-year-olds have it [2]. The 40% of humanity without HSV-1 are largely under 30. Your typical astronaut is much more likely to be towards the 40-50 year old end of the scale.

[1] https://www.merckmanuals.com/professional/pediatrics/infecti...

[2] https://www.hopkinsmedicine.org/healthlibrary/conditions/adu...