Both ran out of stock and both Sony and Microsoft felt it was more important to focus on manufacturing the new consoles than keeping up with the new increased demand for the old ones. Also the Switch was in the peak of its life while the other two were near the end and declining so the former had a much bigger boost potential since a lot of people who have bought a Switch during the pandemic will have already had a PS4 or Xbox One and a lot of people who had none of them probably picked the Switch over the other two since it was more appealing to them, for example a lot of the women who bought a Switch.

Pretty much this. It was the replacement year for the PS4 & XBO. There was only so much stock to go around. For example, Sony only shipped 6.2M units during the 2020 calendar year, with maybe a third of those going to North America. Combined with remaining stock from 2019, there was no way it could have continued selling what it was doing in the spring and summer for the whole year. Even without the pandemic, it still would have probably sold about the same as it did for 2020 as a whole (somewhere in the 2.1-2.3M range; we're still missing Nov.+Dec. NPD sales for last-gen). If Sony had doubled their PS4 output, they might have been able to sell upwards of 3.5M or more, but there was no point in doing so with the PS5 on the horizon.

But the data is clear. There was significantly increased demand for all systems during the pandemic. Prior to when they depleted their stock, the PS4 & XBO were performing far better than they had any right to be doing, even after the initial spike in March-April.

In the six months from March to August of last year, the PS4 alone sold 1.43M, not too far short of the 1.5M the 360 & PS3 sold combined in the same period in 2013. While the PS4 was down 9.5% overall YoY for the May-Sept. period, that's an incredibly small drop for a replacement year, far less than the YoY drops experienced by the 360 (-42.2%) and PS3 (-30%) in the same period in 2013, or by the PS2 (-19.1%) in the same period in 2007. But by October the PS4 had burned through most of its stock, and sales dropped off significantly (and were already slowing down in September). Still, it had sold 1.85M through the first three quarters of 2020, the second-best Q1-Q3 performance in a replacement year ever in the U.S., behind only the PS2, and marginally up from the same period in 2019.

The XBO meanwhile burned through its stock quicker, but was still posting far better than expected sales prior to that. In addition to the big gains in the March-May period, its sales in June, while down YoY, still had a smaller drop-off than the 360 & PS3 did in June 2013. But stock was depleted by July, and while it did get some restocks, it wasn't enough to put it where it was in the spring.

Overall, though, the PS4 & XBO had an outstanding first half of the year, and their performance is clearly due to the effects of the pandemic. To put it all in pictures:

Note that the PS3 and (to a lesser extent) the 360 got one last bump in 2013 before the PS4 & XBO were released thanks to GTA5 in September, plus the 360 got some good holiday bundles in October, which is why their sales were a bit better in September & October than in the five months prior.

The Switch was down in Jan. & Feb. in the U.S., and perhaps also in Europe. It was only clearly up in Japan, likely due to the residual effects of the Lite (see my post above).

AC was going to be a system-seller, but nearly all of its effects would have been felt in March. Aside from maybe the Wii U improving in 2015 in the months following Splatoon's release, there's no good evidence that a single game can cause an increase in baseline sales lasting for months on end, and certainly nothing like a 100% increase.

Nothing ever experiences growth like what we saw without some kind of major stimulative factor. Only price cuts and new models have ever been able to do what we saw. Since there was no price cut and the Lite wasn't enough to explain the growth, and there's no way AC could have done that by itself, the pandemic had to be the cause of the overwhelming amount of the growth we've seen. Data for the PS4 & XBO corroborate the impact the pandemic had on hardware sales.

The data suggests that, absent the pandemic, the Switch would have been at best slightly up in 2021. It wouldn't have sold anywhere close to what it did in 2020 had COVID never been a thing.

Yeah no, the pandemic is the sole contributing factor, others were not responsible at all.

See how your line of argumentation works?

I wonder how smart you felt when typing your reply..

Pretty much this. It was the replacement year for the PS4 & XBO. There was only so much stock to go around. For example, Sony only shipped 6.2M units during the 2020 calendar year, with maybe a third of those going to North America. Combined with remaining stock from 2019, there was no way it could have continued selling what it was doing in the spring and summer for the whole year. Even without the pandemic, it still would have probably sold about the same as it did for 2020 as a whole (somewhere in the 2.1-2.3M range; we're still missing Nov.+Dec. NPD sales for last-gen). If Sony had doubled their PS4 output, they might have been able to sell upwards of 3.5M or more, but there was no point in doing so with the PS5 on the horizon.

Also, a greater share of Switch units are sold to/for kids.  With everyone stuck at home, parents are happy to buy the kids a Switch to give them something to do that (this is important) doesn't require a TV.  No kids fighting over the XB/PS and TV if they each have a Switch.  No parents having to listen to kids gaminh while they're working from home if they can tell the kid to go play Switch in his room.  

Add this to the consoles lifecycles, and Switch was just in a much better position to gain market share during the pandemic.  

Wii also only had one thing going for it. A novelty. Novelties wear off. The switch has a practical use for its selling point. It's convenient having the ability to be both docked and handheld. As HDTVs became more common and Smartphones the less sense Wii made. Switch released just before the cloud gaming boom and it's more practical than cloud gaming IMO. I'm not saying this is the sole factor. Just an additional one to what has already been mentioned.

So were Nintendo's other home consoles with shorter generations and less successful than the Wii worse novelties?