TF39 bypass ratio
GE90 and Trent 900 supposedly have low SFC because they have higher bypass ratio than Trent 800 and other older high-bypass turbofans.
But the original high bypass turbofan, TF39, is supposed to have a bypass ratio as big as 8:1.
Why were next turbofans like TF39 derivative CF6 designed with low bypass ratios like 5:1...6:1? What was wrong with TF39?