What’s the story with the 739ER…looks like it might be a white elephant like the 764!!!

The 900ER compliments the 737 family by allowing an airline to have that little bit more capacity, without having to buy a whole new aircraft type, should they need it.
It would best be described as a specialist model in a versatile family of aircraft. It’s purpose is not to be a mainstream model or a milk cow, so to speak.