AFAIK Kettles in most micro breweries are still copper. Although not a brewer, my chemist dad says it helps to bind sulfites. But mostly copper is valued for it's heat conductivity properties, ie, the heat spreads out nicely instead of concentrating anywhere which can cause scorching. Aluminum has similar properties, and many people use it to brew. Stainless tends to be less optimum in this regard which is why extract recipes make you turn off the heat when adding things.
The reason they used copper "back then" is because they had two choices, copper or iron and I do believe iron is not conducive to tasty brewing.
I have been using copper for my extract brews, and they taste great. Going all grain I will have to use a converted keg though as finding a 10+G copper kettle would obliterate the budget.