There's some truth to this I think.
In the old days the Game Boy really couldn't replicate most of the console Nintendo games. You couldn't have a 3D Mario or a proper Smash Brothers or a 3D Mario Kart back in those days.
Nowadays the handheld line has pretty much all the same franchises maybe with the exception of 3D Zelda, which isn't as important as it used to be.
People like Nintendo games, but Nintendo is pushing it by asking people to buy two systems just to buy games from 1 company (at a sum total of $470 or so for both systems). I think some people are looking at that and saying "y'know ... Mario Kart 8 looks like fun, but do I really need that? I already have MK7 for the 3DS, I think I'm good, gonna spend that money elsewhere".
The Wii + DS succeeded but the Wii was really very different from anything on the market at that time, those types of ideas don't come around often, and in the Wii's case the excitement over motion gaming fizzled after about 4 years of it.
The 3DS does a very good job for most people of scratching their Nintendo "itch" as it were. Even though the graphics aren't as nice, that's never been the draw for Nintendo IP anyway, 3DS can do adequate 3D graphics (unlike the DS).