TPCi stated the reasons for the bans in their announcement:
Control decks and mill decks (strategies that try to discard all the cards from the opponent’s deck) have a place in the Pokémon TCG, but when they become too powerful, the play environment becomes less enjoyable. Bellelba & Brycen-Man has been identified as the card that makes these decks highly effective in tournament play. In an effort to reduce the strength of control decks and mill decks, Bellelba & Brycen-Man has been banned.
Hand destruction decks can be similarly unenjoyable to play against when they consistently achieve their goal. With the addition of Crobat V in Sword & Shield—Darkness Ablaze, there are some hand destruction combos with Mismagius, Dusk Stone, and Reset Stamp that can be achieved more consistently on the first or second turn of the game. In an effort to break up these combos, Mismagius has been banned.
In an unprecedented move, Creatures will re-release both cards with updated effects and artwork. Creatures showed the illustrations they’re currently working on. The new cards will be distributed to players at Gym tournaments after October. They’ll be printed with regulation mark “C” like the original cards even though the current block is “D.” This means they’ll rotate out sooner than new cards.
This move is unprecedented because Pokemon are always just banned and that’s the end of them. Creatures has never decided to “re-release” them with updated effects. So it seems in the future, they may just nerf cards and re-release them rather than outright erase them from the format. They have done this before with Trainer cards like Pokémon Catcher from Emerging Powers, which was re-released with an updated effect several sets later. But this has never occurred with an actual Pokemon before. Presumably they still want Mismagius’s Ability to remain in the format, in some form.