Because of Physics.
Semiconductor mobility depends on the impurity concentrations (including donor and acceptor concentrations), defect concentration, temperature, and electron and hole concentrations. It also depends on the electric field, particularly at high fields when velocity saturation occurs.
Electron mobility is the measure of electron scattering in a semiconductor, also the proportionality factor between the electron drift velocity and electric field as well as between carrier concentration and the conductivity of the semiconductor.
It is measured in unit cm2/V s.
Electron mobility is different for different semiconductors. The electron mobility at 300 K for three key semiconductors: Si -1500 cm2/V s, GaAs -7500 cm2/V s, 6H-SiC -400 cm2/V s.