Metroid isn't exactly that popular and with the exception of Metroid Prime not a franchise where each entry sells several millions. Add to that that Metroid II isn't a hugely popular entry in the franchise and that the remake was released late in the 3DS's life and it's no wonder that the remake didn't sell more than it did.
About the Switch port begging, I get why it's kinda annoying that people ask for a Switch version or use the lack of one as a reason for the remake's performance but at the same time I believe the sales potential would had been significantly higher had there been a Switch version.
Now, I might be alone in this but ever since I started using my Switch frequently there's like a much higher barrier for me to pick up my 3DS, enough to just not care about what looks like a good remake of the only entry in one of my favourite franchises that I never played (I'm not going to count Federation Force). This is also a game that I'd much rather play on a tv screen than on the tiny 3DS screen (I've really gotten used to the bigger screen size of the Switch and it's really hard to go back to a smaller one).
I'll try to not end this in a big rant about my feelings about 3DS exclusives at this point in it's life but while I do understand why Nintendo wants to support the 3DS with more content seeing how it just refuses to die (and I do find that awsome) I don't see how making that content exclusive to the 3DS makes any kind of sense whatsoever.
