The Pokémon Company didn’t say anything about it, but most people assume that:
The upcoming ban likely intends to diversify players’ teams by targeting the most common Pokémon at high levels of play.
The ban targets the top ten most used Pokémon in both Battle Stadium Singles and Doubles. As there are some overlaps, like Cinderace and Dragapult, there is a total of 16 Pokémon banned. Since the top ten most used Pokémon are gone, obviously this would make drastic changes to the metagame and how diverse teams are.
Of course, most likely new Pokémon will become just as used, if not more, now that their checks might be gone. A good example is Urshifu: Rillaboom and Togekiss are both gone now, meaning it could become much more widely used. Could The Pokémon Company have issued this ban to give their newest Pokémon a time to shine? Are they just tired of seeing the same old Pokémon? Are they trying to shake things upon the lull before Crown Tundra? The answer is: We’ll never know for sure unless the Pokémon Company says why.
Hope this helps!