To be fair to NSMB 2, it did wind up selling 12 million WW in the end. Yes, it's a far cry from the 31mil WW from DS and the 30mil WW the Wii had, but the 3DS wasn't able to reach the wider audience/demographics the the DS and Wii had. NSMB DS and Wii also came in at a time when the audience on those systems were already big and diverse. While NSMB 2 released in mid 2012 as the 3DS was still gaining momentum from their drastic price cut from just last year. And NSMBU, well... the Wii U wasn't popular at all to attract much of anyone.
Fatigue is one thing, but this was a drastic drop and I feel there were a lot of factors involved.
not just that but NSMB2 prolly costed nothing.