easy, sngs have lower effective stacks, so quite often you only have to play preflop, and on the flop

cash games have deeper stacks typically, 100bb-200bb, which makes a lot of options more viable (raising turn, raising river, donk bet/3bet on river).

With more decisions, it takes a different skillset to beat. SNG games are more mathematically easy to "solve" because of the smaller stack sizes. THe smaller the stack size is, the more luck there is in a game.