One reason might be that aluminum reacts with sodium hydroxide (lye), which is used in just about every drain cleaning product. "Yep, that 5" hole in the side

One reason might be that aluminum reacts with sodium hydroxide (lye), which is used in just about every drain cleaning product. "Yep, that 5" hole in the side of the pipe sure keeps everything flowing." Heh. The manufacturer could put a warning label on it, but you and I both know how well that's going to work in practice. Edit: oh, and the reaction produces large quantities of hydrogen gas, for extra fun.