Why have so many cards been "banned"?

I recently reopened the game and noticed that my custom decks had cards that had been deemed no longer standard (including my favorite Snoralax-based deck). I was wondering what the reasoning was for so many cards getting banned. Is it because of their release date or to rebalance battles?
Answers
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A lot of your cards, which are from too old expansions, have been rotated, not banned. So some of your rotated cards can still be used in the Expanded format, not the Standard one.
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Yeah, the Pokémon company rotates cards so that only some are tournament legal at a time. That way we don't always have people using the same OP cards over and over, like Shadow Rider Calyrex Vmax for example. Plus, it forces people to try new cards, which has the (probably) intentional side-effect of more cards being purchased.
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