Making the original game was a big budget endeavour, but porting an existing game is much cheaper than creating one from scratch. And while yeah, bringing Doom to Switch would've been more challenging than bringing it to Xbox One, it's also not a case like the 2008-2011 COD titles on Wii where the game basically had to be completely rebuilt.
And there are also third party games on Switch that have sold far more than Doom, like Skyrim and FIFA.
Its possible Activision did the Wii ports with higher expectations. I mean CoD3 was apparently the first and best selling. Wii was essentially the only platform where CoD sales decreased during that period you mentioned, but I could be wrong.
Its evident porting games PS4 to Switch is big endeavor compared to PS4 to X1. And I'm sure that's always considered when doing anything for Switch. Frankly, some ports to Switch are quite poor and I'm guessing it requires more resources/money/effort for a good Switch port.
Skyrim and FIFA did do exceptionally well on Switch, that support will likely continue to some degree. Maybe Bethesda's priority should be bringing Fallout games to Switch as opposed to more id shooter. FIFA is among the most popular IPs, but the Switch port is using the 7th gen engine and so will the next game it seems. Hence, EA isn't putting a big effort in the Switch port yet.
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