In terms of preparing against your opponent, at their level and with enough time to think over the board, one has to know so much theory about many lines, that as white you're forced to find "novelties", which are brand new or mostly never played lines that might never come up in the game, in the hopes of throwing your opponent of their preparation. With black, you have to prepare a few opening lines and know them so well as to avoid surprises and maximize chances of at least a draw. This takes a lot of time and energy and memorization becames a huge factor.
When you face many opponents, the ROI of deep studying a few lines doesn't pay off against people who have a myriad of styles, so memorization plays a lesser role; and you play many tournaments so having a few bad ones isn't such a big deal, comparing to playing for WC.
