Why is this the maximum dynamic range in a spectrum analyzer?
Signal to noise ratio gets worse as signal level falls.

Signal to distortion ratio get worse as signal level rises.

Dynamic range is the ratio of the signal level to any unwanted signal, noise or distortion.

Therefore there is a signal level where these two unwanted signals are equal. At either side of that level, one or the other unwanted signal gets worse and dominates. At that level, dynamic range is best.

Note that the noise level depends on the resolution bandwidth, whereas the distortion level doesn't. The optimum dynamic range level therefore changes with bandwidth.