Why does an over in Cricket have exactly 6 balls? Any logic behind that?
There was a Cricinfo article which discussed the number of balls in an over, and which suggested (emphasis added):
There is no recorded reason for the moves from four balls to five and from five to six. But it is understood that the shortness of four- and five-ball overs - which meant too many over changes in the course of a day, and also did not afford bowlers too much leeway in terms of building up a rhythm and plotting a dismissal - contributed to their demise. [...] Again there is no recorded official reason for the move to six balls after 1978-79, but it is widely believed that with the commercialisation of the sport and post the Kerry Packer revolution, there was no room for the eight-ball over, and the six-ball over was a happy balance.
With that in mind, I'm inclined to think that this question is not definitively answerable to any further extent. The length of the over appears to be partially arbitrary (it has to be some length) and partially pragmatic (it's about the right length to allow a good contest between bat and ball).
There were 4 and 5 ball overs at different stages. The problem with these is it takes a long time to change the field at the end of each over so when you bowl 130 overs in a day it is a lot of unnecessary messing about.
8 balls is a lot for a fast bowler to manage without a break in a Test match..