The 2018 model was released to counter hacking.
New information has come to light following Nintendo of America’s lawsuit against the hacker Gary Bowser last year and the publication of the court documents. In it Nintendo officially confirmed that the new model of the Switch hardware released in 2018 was a direct result of trying to prevent piracy:
Nintendo has had to release a new version of our hardware in response to one of these hacking tools, and this modification entailed countless hours of engineering and adjustments to our global manufacturing and distribution chains and, of course, corresponding resources. To be clear, these effects are a direct result of the defendant and Team Xecutor attacking our technological protection measures.