Why is the thrust-specific fuel consumption much worse for high bypass turbofans at cruise?
Yes, that is typical for high bypass ratio (HBPR) turbofans.


  But why?


Because the entry impulse goes up while the exit impulse stays roughly constant. Thrust is the difference between both, derived over time. 
The moving engine needs to slow down the airflow for combustion to take place, and then needs to accelerate the air by more than it has been slowed down to have positive thrust. Hence, SFC goes up in parallel with speed.

While the internal process inside the engine in flight is very similar to that at rest (only the pressure levels are increased by the ram effect of the moving engine and density drops with increasing altitude), thrust is reduced due to the smaller impulse difference in flight. Since a HBPR engine accelerates most of the air volume streaming through it by only a little, the average exit speed is relatively low compared to a jet engine with no bypass flow. This is similar (but less severe) to a propeller where thrust drops with the inverse of speed.

SFC gets even worse in supersonic flight. For a meaningful SFC comparison speed needs to be the same.



Thrust-specific fuel consumption in g/kN·s of a turbofan engine with bypass ratio 3. Y axis shows altitude in Meters. Isolines wer made with R contour function and should be more rounded in reality.