I'll tell you the true reason. Over the years, core gamers have just lost interest in Nintendo's home consoles. The same old IPs, even with slightly different takes on them, is not enough for them. They went elsewhere for different IPs and experiences. This only leaves the big Nintendo fans to buy their systems, which there seems to be maybe about 18M or so. Unless the NX is WAY better than the PS5, I see the same fate for it.
I think MS is starting to see the fruits of pumping out the same old franchises, too. Sure, they have a couple new IPs coming, but they made sure to get their big hitters out, first. Again, this really only helps with the big Xbox fans. Fortunately for MS, they still have good 3rd party support.
The market Nintendo has always had cornered, was filled with casuals to begin with, which is why they have always been successful there. Unfortunately, the casuals are starting to switch to smartphones. This is why the 3DS will fall incredibly short of the DS, even though it has little direct competition from something like the Vita.
