Why would one want more than 40 cards in their deck?
Decks relying on "That Grass Looks Greener" want much more than 40 cards

One deck archetype that wants to have more than 40 cards are "lawnmower" decks, named after the key card That Grass Looks Greener.


  If you have more cards in your Deck than your opponent does: Send cards from the top of your Deck to the Graveyard so you have the same number of cards in the Deck as your opponent.


So if you have a 60 card deck and you play against an opponent that has a 40 card deck, you can expect to send around 20 cards to the graveyard. If your deck relies on archetypes that benefit from having cards in your graveyard (e.g. Zombie, Lightsworn, Infernoid), then that can put you at a huge advantage.

According to this Reddit post, decks based around "That Grass Looks Greener" (and decks designed to counter it by having 60 cards) were popular from February to May 2017 to the point where a number of top-level decks were 60 cards. Konami made the card Limited in the TCG to discourage this (although it's still Unlimited in the OCG). However, a lot of people seem to view it as a fad deck that would have lost its appeal over time even without a ban.