I have a strong feeling, in the case of Fallout 76 it has to do a lot with network. But yeah, quite some games should be difficult or impossible.
Fallout 76 uses basically the same antiquated engine as Fallout 4, which struggles on PS4, XBO, and even many PCs. It bogs down with the newer techniques and solutions that have been spliced into it. Shadow rendering, for instance, makes my PC cry tears of blood in downtown Boston during bright daylight. I just can't imagine Fallout 4 being playable on the Switch and Fallout 76 most likely falls into that same category for the same reasons.
There is a Chinese proverb quoted by the Captain of the Yangtze nuclear submarine in Fallout 4 when he says, "They say a shipwright should not build new ship out of old wood. But we have no other choice." I can't help but wonder if that was a message from the developers.