Bethesda has explained that Fallout 76 couldn't come to Nintendo Switch because, after consideration, "it just wasn't doable".
Speaking at PAX Aus (as reported by GameSpot), Bethesda's Pete Hines explained:
"The Switch is something I can say with certainty that... it's a part of every conversation with every dev we have now about what we're doing going forward, because we consider it to be a viable platform. If the game will work on it, we want it to be on every platform possible. Fallout 76 is not because it just wasn't doable."
No detail was given as to exactly what wasn't doable about a Switch version, but the always-online nature of the game and absolutely gigantic world probably had something to do with it.