Because that trend can translate in less people adapting to consolegaming from an early age,an age that those people normally get a nintendo console and when being older they just immidiately jump to the more mature console being the PS4 so that could explain PS4's gangbuster sales in comparison.
Just speculation ofcourse. :p
Edit: Although the switch sales look fine to me,also worth noting they come from the lesser userbase that was the WII U.
That's horse shit and you know it. Nintendo themselves revealed the biggest group of switch buyers is 20-35 year olds. The switch is made for the age group you're describing
Just a guy who doesn't want to be bored. Also