Why is the deciding set of volleyball played to 15 pts. only?

Question

Why is the deciding set of volleyball played to 15 pts. only?

Answer 1

 in college, it used to be every set played to 15 side out scoring except for the fifth set which used rally point scoring, so they changed the first four to rally to 25.  (Imagine the fifth set as "OVERTIME").  

in lower level club, it was similarly sideout to 15 first two sets, rally to 15 third set.

now everything's rally.