In Jeppesen approach plates we see that the Transition Altitude of the aerodrome is fixed but the Transition Level is not fixed and is given by ATC. What is the reason for this?
The point of flying on QNH is to keep you from flying into terrain, which is at a constant altitude above sea level, so TA is fixed.
Once you are high enough that terrain can’t be a problem and you only have to worry about hitting other planes, everyone can transition to QNE. However, because TL must be physically above TA, the difference between QNH and QNE will dictate what the lowest usable flight level is.