Because 3m feels like the bar for a game to get an in-house developed sequel -- unless it's a Miyamoto pet project like Pikmin or a really low budget game like Rhythm Heaven.
I guess I just don't see it that way.
Doing 3 million lifetime is pretty meh when you consider how early Arms released in the Switch's life cycle. It gets even worse when you consider the huge boost that just about every Nintendo franchise has had on Switch (just look at how Kirby, even without being a critical darling, blew past expectations). Arms is not doing very good at all ... it's just doing "ok", and it seems like that's not something most platform holders are happy with.
Don't get me wrong, I do expect an Arms 2. I just wouldn't be shocked if we didn't get one, and I don't think they'll make an Arms 2 because of sales (if that makes any sense...)