Not only because cards have realistically gotten as powerful as they can without becoming laughable, but because the Pokmon TCG has lost a lot of the strategy of lower-HP eras in favour of cage fights that are over in minutes
Looking for Mega Gengar ex prices and values
You actually get more value running it in Pioneer, Historic, or other formats that allow up to four copies of a valid card in the deck
Though you can store cards in mini tins, they are by far the smallest type of tin available and, as such, wont hold much of your bulk
Take the established one-mana 2/1 statline, add a dash of dash, and throw in an ability that, without exaggerating, can swing the outcome of a game on turn one, and you've got an egregiously overpowered one-drop
Now she does cost you 5 starting life, so that's 15 outside of Commander