Question

why are there only 6 balls in an over

Answer 1

When cricket returned to England, they reverted to the six-ball rule. South Africa used eight-ball overs from 1938-39 to 1957-58, and Pakistan from 1974-75 to 1977-78. Even New Zealand went back to the six-ball overwhen cricket resumed after the war.

Answer 2

It's cricket rule, there is not any reason just like you brush everyday.