The efficiency of a wind turbine can be significantly reduced if it is mounted either too low or too high above the ground.  In some areas, winds tend to have a greater velocity at higher altitudes while in other areas winds tend to have a greater velocity at lower altitudes.  So, thoughtful engineers position turbines at heights where average, long term, wind velocities are greatest, all other design factors being considered equal. 
Regardless of long term trends, from day to day, and in some cases from hour to hour, the relative velocities of winds vary at different heights above the ground.  For example, during the morning, near-surface winds are strongest at some locations whereas winds well above the ground are strongest during the evening.  A turbine positioned at a fixed elevation cannot benefit from these fluctuations and, in fact, is impeded by them.