As more cards are released, 60 card decks become more viable, because there are naturally going to be more cards that play well in your strategy. When the pool of good cards is limited at the initial release of the game, you want a smaller deck so that you arent diluting your good cards with mediocre or trash ones and your draws are more consistent. But eventually, you see that more and more cards get released that do similar things to cards you already utilize and so the dilution effect isnt as pronounced. Still, if your deck strategy hinges on pulling a specific card by a certain turn, and you can obviously only have 3 copies of that card in deck, the larger your deck is the more unlikely your strategy will work out.