All engines have compromise elements.
They can produce power over a wide rev range, but not be so efficient at doing that, or produce power within a relatively small rev range, with greater efficiency.
Turbines, particularly, are examples of the latter. They're a bit like "peaky" two stroke engines. Very narrow power band, fairly inefficient (and gutless) outside that power band.
Since a lot of lighties spend significant portions of their operation at other than near-maximum power, the inefficiency becomes significant.
Add to that the inefficiency of running the jet at low levels and speeds, and it compounds.