It is a very similar sin to that the pharisees committed. I will paraphrase Jesus here but he told them that "you are like whitewashed tombs. Who outwardly appear pure and holy but inside are full of all uncleanliness." It is because of the sins of pride and hypocrisy. It also is trying to bring attentionto yourself above others, so again, pride. We shouldn't present ourselves to the lord as anything more than we are, however, we should still dress nicely for church to show respect to God. But if you dont feel convicted about wearing fancy clothes, then go ahead, just dont do it in a way that causes you to sin