Maybe because it's non-magnetic and so won't suck the magnet in the phono cartridge down into it. Just surmising here. Stainless steel would be more expensive to buy and machine, glass is freqently used, as are the plastics. But I don't know that all of those don't come with their own issues. I dislike the sound of plastics in particular.
Why Use Aluminum for a TT Platter?
Mass I am told is a good thing when it comes to TT platters. Lloyd Walker for one extolls it's virtues and as a rule some of the better turntables like to brag about their big ..Platters. Why then would aluminum, known for it's light weight (low density) turn up as frequently as it does as a platter material. I know it is easily machined but isn't there anything better and much denser.