Nintendo was better insulated against the rise of smartphones/tablets at least initially, because as of really 2013, it wasn't easy to get a budget tablet and phones are contract devices so most kids don't have those.
The iPad was for its first couple of years a luxury device out of reach of kids and there weren't a ton of tablet knock offs, but by 2013 cheap Android tablets started to flood the market.
Today even kids are getting their own tablet because they're actually cheaper or the same price as the 3DS.
Think of smartphone/tablets as an earthquake + tsunami ... Sony was closer to the epicenter of the earthquake and thus got rocked more immediately, but Nintendo also got hit later.
Mobile first invaded the adult market then quickly after the teen market, now it's reaching the kids market too, so Sony which targets more of the teenager/college age audience was naturally rocked the hardest first.
