Question

Why fan rotate anti clockwise why not clockwise

Answer 1

The fan rotates both in clockwise and anti-clockwise directions. The direction depends on the way of electricity fed to the fan.

Answer 2

All fans can rotate both clockwise and counter clockwise depending on which way the electricity is fed to them. Depending on which way the fins on the fan are tilted relative to your view and which way they are turning, the fan will either suck air or blow air. Here are the possibilities: 

1) fins twist clockwise in shape as they stretch away from you, fan spins clockwise: blows air 
2) fins twist clockwise in shape as they stretch away from you, fan spins counter clockwise: sucks air 
3) fins twist counter clockwise in shape as they stretch away from you, fan spins counter clockwise: blows air 
4) fins twist counter clockwise in shape as they stretch away from you, fan spins clockwise: sucks air 

So, there is not one right way to build a fan. There are two ways to build sucking fans and two ways to build blowing fans (also, making a blowing fan into a sucking fan is easy - just flip a switch or turn the fan around). So the direction that the fan spins depends on the taste of the designer in which option he chooses.