I believe the "real" reason is that Maxie's Hidden Ball Trick has causing issues for a while. The latest releases just made it crystal clear problems were going to continue popping up. Possibly, given the very high standards Japan seems to have for getting a card banned, I almost wonder if they purposely released two Pokémon in SM9 - Omastar and Kabutops - that would "cross the line" and justify the ban in most fans eyes.
It helps to understand the last time Maxie's Hidden Ball Trick dodged the ban-list-bullet. A general rule of Pokémon card design is that some effects are balanced out by not being available on either player's first turn. Whether this is because the designers think Evolutions need the "help" of stronger effects or because it really is as simple as "This is broken T1 but not later.", that's how it is.
If you're not familiar with the Expanded Format, Ultra Ball was (and I think still is) the go to Pokémon search method. VS Seeker is pretty much a four-per-deck staple. Battle Compressor usage was near universal back then, but is still reasonably common now - adding it to a deck that wasn't using it is a fairly simple feat. Why do these matter? They facilitate running just one or two copies of Maxie's Hidden Ball Trick, plus one or three [F] Type Pokémon for Maxie's Hidden Ball Trick to Bench, in any deck that can spare as few as two slots!
Which is where Archeops (BW - Noble Victories 67/101; BW - Dark Explorers 110/108) and Gallade (XY - BREAKthrough 84/162). I vaguely recall there being a third popular [F] Type Evolution which sometimes joined them, but if your opponent was running a deck built around Evolutions and you went first, you could try for a relatively easy hard lock on them playing Evolution cards from their hands. Yeah, there were ways around it, like running Evosoda or (before he was banned) Wally, but you were still in bad shape if your deck required multiple Evolutions once Archeops hit the field and its "Ancient Power" prevented you from manually Evolving your Pokémon. Gallade provided a solid, single-Prize [F] Type attacker, easily fueled by just a Double Colorless Energy while also using its "Premonition" to improve your draws.
You'd see this trick in almost every deck that could make room for it... which meant most beatdown decks built around Basic Pokémon. Many could spare not just two slots, for the minimal version, but five or six, for the "I still can pull it off if one piece is Prized!" variety. Archeops was banned for this reason; I don't think it will return as even with both Maxie's Hidden Ball Trick and Wally gone, now we've got Ditto {*}; one turn slower but arguably easier to implement. As for Maxie's Hidden Ball Trick's new partners, Omastar can enable a T1 Item lock if your Bench is smaller than your opponent's while Kabutops can enable T1 Supporter lock if it is your Active. Having a smaller Bench can be a real sticking point when you're trying to pull off a T1 Maxie's Hidden Ball Trick - it is usually supplemented with cards like Shaymin-EX (XY - Roaring Skies 7/108, 77a/108, 106/108) so filling your Bench while going for the combo is pretty common. Obviously, you cannot lock down both aspects of the game at once but...
...imagine what the combo could now do, slipping into various beatdown decks.
If you can spare the time, I'll explain in some detail.
