Plenty of reasons, one of them is if you play usually a type of game, you won't be as good as on an other one, if you play cash like you play mtts for sure you'll be losing, you will tend to play too loose, because in mtts you have to as the blinds grow, while in cash games, patience is key. There is a psychological reason there, you play directly money, so you have to be ready to lose money directly even with winning hands, because if you tilt you get to lose way more afterwards, while in mtts, you will lose your buy in at max (unless you tend to buy in other mtts at tilt, but it's less likely). Unless you play rush&cash or zoom cash, you have to keep the focus on other players' plays, and take reads, it has way more value, as you can end up playing hours with same ppl, while in mtts that happens usually at the final table, where you are usually short stake. The variance is hagher in mtts, so if you are more winning it may be because you are just more lucky at the moment, the edge will be decided on limitting your losing streaks, and getting a higher value on winning ones.