How do aircraft engine manufacturers achieve a higher bypass ratio while still meeting the thrust requirements for a given aircraft?
Early turbojets were so inefficient that adding a fan was considered but not implemented because that would had made the engines even more sluggish and narrowed the operating limits even more. If you compare the Jumo 004 with the EJ200, you will find that both are of similar size, have  8 compressor stages, but vary widely in their compression ratio and thrust (3.2 vs. 26 compression ratio and 9 vs. 60 kN dry thrust).

When compressor flow was better understood, turbine temperatures and pressure ratios rose, the excess power delivered by the engine core rose and allowed to drive a second turbine and shaft; initially with bypass ratios as low as 0.25. If you compare the overall pressure ratio with the bypass ratio, you will certainly spot a trend:

Engine             compressor       pressure ratio     bypass ratio
RR Conway        7LP, 9HP stages       14  :1             0.25:1
P&amp;W JT3D      2fan, 6LP, 7HP stages    12.5:1             1.42:1
GE CF6-6      1 fan, 1LP, 16HP stages  25  :1             5.8 :1
RR RB211-535  1 fan, 6IP, 6HP stages   25.8:1             4.3 :1
GE 90         1 fan, 4LP, 9HP stages   42  :1             9   :1
RR Trent XWB  1 fan, 8IP, 6HP stages   52  :1             9.3 :1
RR Ultra Fan                          &gt;70  :1           &gt;15   :1 (projected)


A more efficient core allows to leave more energy in the exhaust past the high pressure turbine, so the low pressure turbine can extract more power to drive a larger fan.