There are several factors. First of all, a shower is a small, enclosed space, so the fart gas is more concentrated, and the high turbidity of the air in the shower circulates the gas through the space effectively. Secondly, the high humidity and high temperature conditions in the shower enhance a person's sense of smell and taste. The farts don't actually smell worse, it's just that we can smell them better than usual. Similar conditions prevail in the bathtub.