you're comparing Metroid to franchises that release on mutliple platforms. If Metroid had been doing that it would probably have 2x the amount of sales for each of the last few installments.
Selling 1 or 2 million (the Prime installments) on a Nintendo console is fairly solid. It is somewhat niche because no child is going to play a Metroid game (well under the age of like 8-10), so I wouldn't consider them having done that poor
especially when you consider that half of those titles (look at thines like Fusion or Zero Mission) were extremely easy to develop and in terms of development costs
something like Metroid on a Nintendo console selling a million or two is pretty good when you consider that a large amount of the fanbase are more into party/ releaxed platforming games
