The difference I see here is because almost no decks need items to setup and because every deck use items, so this kind of lock would affect most decks the same way, while there are decks that need abilities to play, since that many decks that need the Shaymin-EX/Octillery to draw power, the new Omastar deck, Greninja BREAK and some others.
I must disagree. I am not trying to play word games, but yeah we are using things like "need" rather differently. If I use it strictly, where so long as I can get something up and attacking without Items, then your are correct, Calcab. However if we apply this same standard to most decks, they also do not need Abilities. Even Greninja BREAK can get up and attacking without them.
What matters is a "proper" or "competitive" set up, which takes into account a deck's strategy as well as pace. Without Items, very few decks can get a good, solid early game that transitions into a similarly worthwhile mid game. Most decks rely heavily on VS Seeker and Ultra Ball early game, with many also relying on Battle Compressor. Their set up is incomplete as well if they lack the desired Pokémon Tool. Without Abilities, many decks are also diminished, however it is because they lack access to Shaymin-EX (ROS). Which brings us back to the debate about how decks are built. If your deck is so Ability reliant that you cannot even get set up without Abilities, and what little you can do isn't worth it, then at least accept this is not how most decks operate. The three core kinds of cards are Pokémon, Trainers, and Energy. Items are one of the three kinds of Trainer cards. Abilities are not one of the kinds of Pokémon; they are one of the two kinds of effects you'll find on Pokémon (barring gimmicks like Ancient Traits).
The more we discuss this, the more I am starting to wonder if we would indeed be better off having Abilities become a much less important part of the game. The main reason I do not is that perhaps the biggest offender (Shaymin-EX) isn't going to care; instead decks using Garbodor will most likely drop a Shaymin-EX or two before locking down Abilities, something not uncommon when facing them now.