I think most people are already familiar with the points I'm going to bring up and probably also agree with the statement, but I'm going to add a point I've yet to see mentioned on this subreddit. This is a longer post, but there is a TL;DR at the bottom if you don't want to read it all.
Let's start simple; here's why Pascal is better than Maxwell hardware/specs wise.
Pascal doesn't necessarily indicate better performance, as a matter of fact it offers minimal upgrades in flops. However, it's far more efficient concerning heat dissipation, power consumption and size. It's an ideal upgrade over Maxwell for Nintendo to use, especially considering the form-factor of it being a handheld and home console.
This allows for better performance both while docked and undocked, especially undocked since there would be no need to sacrifice battery life while providing better performance in theory, with no risk of thermal throttling.
We also have the fact Pascal would be cheaper for Nintendo to use, since nVidia is going to replace Maxwell with Pascal and manufacture chips for Tesla's cars and other similar projects including a revised version of their own nVidia Shield TV (more on that later), so nVidia would have to manufacture chips that use a different (& outdated) architecture separately which would cost more, therefor contrary to popular belief, it wouldn't be cheaper for Nintendo to use Maxwell.
Now, let's look at it from nVidia's perspective. They dominate the PC Gaming scene, but not at all in the console gaming department. With everyone switching to AMD they decided to release their own set of consoles, but we're going to focus on the Shield TV.
It's generally not considered that much of a success, and since it's lacking in the serious gaming department (a.k.a the library of games), and it's often referred to as an Android streaming box (with some gaming functionality) because of this. Sure, it's the best Android TV device on the market (though quite pricey compared compared to other options), but it didn't manage to prove itself where it meant to, for example;
When it was announced, it was shown playing Crysis 3 and Borderlands, with Borderlands only released around October last year (a year after it's announcement), and Crysis 3 still not released (and most likely won't be). This indicates enthusiasm from nVidia and their plan to provide console quality titles on an ARM Android device cheaper than other consoles using their own hardware, a great way to attract more people, make a profit off your new chips and showcase your new fancy tech. But, the AAA titles never came, and with the only other games being ones you can already play on your phone (and were optimized for such), it never managed to impress in the gaming scene.
So, they couldn't get their hardware in any current-gen console and their own consoles didn't really take off because of a lacking library of exclusives or console-quality games. There have also been reports/rumors of nVidia's investors being dissapointed in their lack of presence in the console gaming industry, indicating they were forced/pushed to do something about this issue.
Back at Nintendo, the 3DS was a moderate success and the Wii U flopped hard, they start working on a new console, they decide that by merging divisions to develop for one hybrid console there would be no droughts of 1st party titles. Now you need hardware powerful enough to offer the same experience when you're on the go and when you're playing at home, but also powerful enough for 3rd party devs to port games, making the ultimate yet affordable gaming device.
Back at nVidia, they are trying to get into the console gaming industry and offer their hardware, but since both Sony and Microsoft are already using AMD for their consoles, they turn to Nintendo, where they have the ideal form factor for showcasing their new Pascal chips and also a another way to make profit off them (since they're already going to be used in other projects).
TL;DR ~ With nVidia failing to provide hardware for the big three and get into console gaming, they try but fail with their own gaming devices, with Nintendo's decision to create a hybrid console and have both hardware divisions pumping out 1st party titles they need competent ARM chips to provide enough power for 3rd party devs and provide the same expirience at home or on the go, with nVidia having the most powerful ARM chips on the market and Pascal also having perfect attributes for the Nintendo Switch, it's a match made in heaven. Using Maxwell is illogical for both, and therefor most likely won't be used.