Maybe it's the water and mild abrasives you are introducing via excessive polishing. or maybe it's because modern Harley aftermarket stuff is made in China and Taiwan, and their chrome is not nearly as nice as ours was. Stuff chromed in Mexico is nicer. And not to get all eco-concious but chroming is a very toxic procedure, which is why it's so expensive.
I've never understood chrome plated aluminum, my looptail "P.K.Ripper" was plenty shiny.