Dress codes serve as class signifiers. Like most rules of decorum, they're cultural artifacts used to delineate the haves from the have-nots. They don't dislike the fact that Fetterman refuses to wear a suit. They dislike the fact that he dresses like the common people he actually represents. Whereas they dress like the people they represent - capitalist oligarchs. They're wanting to close ranks and keep people from realizing that not everyone in the senate serves the same masters.