Why are there so many different methods of performance appraisals?

This is a
great question. There are many different methods of measuring performance for several reasons.
Let me give you a few of the nontechnical ones from a commonsense point of view. 

First, it is not easy to measure performance. So, the more a person has to measure
performance, the more accurately performance can be measured. For example, when measuring
performance, do you measure productivity, speed, wise decision making or something
else. 

Second, in connection with the above point, different aspects of
performance are measured with each appraisal. Moreover, since all people are different, certain
appraisal are better than others depending on the person, who is being evaluated. 

Finally, there are many different tasks in any workplace. So, to measure each task
requires different appraisals. For more technical information, see the reference link
below.