Interesting. Learn something new everyday. I assume price-wise the command a bit more than steel based?There are several reason why;
They stick in some guns. This is my main reason. I shoot a Ljutic, and they all will not extract to some degree. I also had some stick in an 870, and I ripped a finger open when mortaring the gun to get the hull out.
Though these reasons are less urgent, wear on guns and reloading presses are often cited.
The only brass based hulls still made are Win AA, Remington STS, Nitro 27 and Nitro Gold Sporting, Federal HOA, and Federal papers.