SMB 2 was (is) actually not a very good game, not to mention way too hard. It also looked exactly the same as SMB, and by 1988 gamers were expecting games with better graphics. It would have been disastrous for Nintendo to release it.
As an aside, did you know SMB3 was released in Japan the same year SMB2 was in the US?
EDIT: Oh yeah, I'm looking forward to the next installment of your Bump N Jump review too!