This one was easy to spot.
Initially, the design reason beside the ball (1970 & 1974) black pentagons (such a classic!) was to make the ball more visible to the audience with black & white TVs.
At that time, the penetration of color TVs in the total population was still smooth.
The 1978 World Cup in Argentina shows a design change of the official playing ball, by removing the black pentagons from the previous ball with "triads".
Obviously, there was no need for making sure the ball is more visible to the TV screen (in 1978 the color TV was present in 78% of the US households).
And it went unchanged since 2002, where the South Korea & Japan World Cup playing ball design was out of the ordinary of the last 6 World Cups :
1978 to 1998 = 20 years of lacking inspiration in soccer ball design.
Which didn't prove further, as the next 4 world cups shows us a diversified ball design.
But obviously, all balls are round!
Have a nice day!
Sources:
http://www.soccerballworld.com/
http://www.tvhistory.tv/
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