The Switch is supposed to be the 3DS' successor. And will eventually become THE portable gaming in Japan, I'd say it already is. You may think it's not fair, but that's just the way it goes.
And of course it's sold more because that's the system that has been out the longest and it's a Nintendo handheld AND the successor to the DS.
The Vita was a flop.
The Wii U were dead upon arrival.
The PS4, while doing everything it can and may ultimately outsell the PS3 over there, is not coming close to it.
And the Switch JUST came out.
I'm talking about launch specifically up to this point, and the Switch is neck-and-neck thus far and would be beating the 3DS convincingly if there was enough supply to meet demand.
Hardware Comparison Threads:
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PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017