There are two main reasons:
To fly straight and level at constant speed the thrust must be equal to the drag. Both drag and IAS are proportional to dynamic pressure. So as you increase altitude at constant IAS, the drag remains constant.
But the TAS at any given IAS increases with increasing altitude, such that at 40000 ft the TAS is about twice the IAS. So you get more knots for each pound of thrust.
Secondly jet engines typically give the best specific fuel consumption (most thrust per pound of fuel burned) when running at between 85% and 95% RPM. But at low altitude this RPM range would produce excessive speeds.
As altitude increases the air density and mass airflow through the engine both decrease. This reduces thrust at any given RPM. By climbing to a suitably high altiutude it is possible to fly such that thrust in the optimum engine RPM range matches the drag at the optimum TAS. This turns out be be at about 1.35 VMD.