why zener diode is named like so?

Answer 1

According to this article:

http://en.wikipedia.org/wiki/Diode

It says that they are named for Clarence Zener the inventor.

Answer 2

Zener Diode is named after the discovery of "Zener Effect" (They are named for Dr. Clarence Melvin Zener inventor of the device)
which states,

"With the application of sufficient reverse voltage, a p-n junction will experience a rapid avalanche breakdown and conduct current in the reverse direction. Valence electrons which break free under the influence of the applied electric field can be accelerated enough that they can knock loose other electrons and the subsequent collisions quickly become an avalanche. When this process is taking place, very small changes in voltage can cause very large changes in current. The breakdown process depends upon the applied electric field, so by changing the thickness of the layer to which the voltage is applied, zener diodes can be formed which break down at voltages from about 4 volts to several hundred volts"

Zener Diodes are basically used for voltage Regulation purpose. thus make a very suitable way to have deisred "reverse breakdown value" and use it as an voltage regualator with few more associated circuit components.