Most likely because it allows you to take your entire discard pile and shove it back into your deck.
>It was announced in June 2015 that Lysandre's Trump Card was to be banned from all sanctioned Pokémon TCG tournaments as it was ruled to create an unbalanced playing environment. The specific concerns were:
• It would eliminate the possibility of a player running out of cards in their deck, a victory condition for the other player.
• It allowed repeated use of trainer cards.
• It allowed a player to quickly draw through their deck without any negative effects.
• It significantly increased the length of battles.
~ Bulbapedia
