Re: the OP --
Short answer:
It was a necessary change to mandate a high contrast service environment. This helped correct distortions in the game caused by evolving equipment & changing player strategy.
Long answer:
In the late 70's / early 80's, players at international level started using both tacky & pimples out rubbers (combination bats), with black rubbers on both sides, and also started twiddling the bat under the table during service.
Combined with the advent of other factors (such as the high toss serve, and imparting knuckle spin on the ball with the free hand during the ball toss) this meant the vast bulk of points in a match were being won/lost purely on serve.
This was detrimental to the nature of the game on the whole, as it made game play hugely imbalanced -- it took the focus off of other skills / strategies / game play styles / rallies & tactics. The serve started becoming everything -- coaching and player development pathways were putting emphasis on serving over every other skill in the game, purely in order for a player to simply compete with the better servers.
[NB: it's relevant to point out, a similar thing was also happening in Tennis at the same time, with the growing use of oversized carbon fibre rackets with longer handles. This enabled higher string tensions, making for faster racquets, and serve speeds increased dramatically. Many players were still using wooden racquets at the time, even at the top level... But they were forced to change to the more expensive carbon fibre racquets, as wooden ones simply couldn't compete.
There was huge debate at the time over possibly changing the rules around tennis equipment as a result, as all of a sudden the game was being overtaken by serving masters -- this was detracting from tennis as a spectator sport. TV viewers at the time liked the longer rallies as they add to the game's drama, so they started to switch off. Players adapted over time and TV viewers returned, but the game had also changed forever, some say for the worse -- serve and rally was out, long boring baseline battles were in, taller players came to dominate the game due to the simple mechanical advantages they enjoy during service, and nothing has really changed much in the 40-odd years since].
Since its inception, the ITTF has always wanted TT to be a game that's accessible to everybody, even at the highest levels.
As TT was now trending towards becoming another game like tennis, where service was everything and the most expensive equipment wins, over time the ITTF brought in a host of rules to 'even the odds', bring balance and diversity back to the game, and prevent players from using either more expensive equipment (or a small handfull of techniques) from dominating matches.
These rule changes included the (now traditional) black/red rubber combo, as well as other rules, such as tossing the ball straight up with an open hand during service, and the requirement for a competition blade to have a wooden surface on both sides, and contain a minimum of 85% wood by thickness.
And as for why black and red, it was always about mandating extremely high visual contrast. Players have tried other tactics in the past to disguise the serve, such as stamping or grunting to hide the ball impact sound, combining green rubbers with green and white shirts then serving in front of the body, hiding the impact stroke behind the body during service, and so on.
This is also why balls in ITTF matches are either white or orange, why you can't have white or orange coloured rubbers, why there must be high contrast colours between each side of the bat, and why player uniforms at ITTF tournaments cannot be the same colour as the ball.
